Refactor HTTP serialization (API Change):

A new class `serializer` is introduced to permit incremental
serialization of HTTP messages. Existing free functions are
re-implemented in terms of this new class.

* The BodyReader concept is refined to support a greater variety
of strategies for providing buffers representing the body to
the serialization algorithms.

* Added buffer_body, a new model of Body which allows the caller
to provide a series of owned buffers using their own serialization
loop.

* Added empty_body, a model of Body which is for serialization only,
to represent HTTP messages with no content body.

* Removed overloads of write and async_write which send only
the HTTP header.

* Removed public interfaces for performing low-level chunk encoding.

include/beast/websocket/impl/accept.ipp

@@ -9,6 +9,7 @@
 #define BEAST_WEBSOCKET_IMPL_ACCEPT_IPP
 
 #include <beast/websocket/detail/type_traits.hpp>
+#include <beast/http/empty_body.hpp>
 #include <beast/http/message.hpp>
 #include <beast/http/header_parser.hpp>
 #include <beast/http/read.hpp>
@@ -39,7 +40,8 @@ class stream<NextLayer>::response_op
     {
         bool cont;
         stream<NextLayer>& ws;
-        http::header<false, http::fields> res;
+        http::message<false,
+            http::empty_body, http::fields> res;
         int state = 0;
 
         template<class Fields, class Decorator>
@@ -188,8 +190,8 @@ class stream<NextLayer>::accept_op
 
         template<class Buffers>
         data(Handler& handler, stream<NextLayer>& ws_,
-                Buffers const& buffers,
-                    Decorator const& decorator_)
+            Buffers const& buffers,
+                Decorator const& decorator_)
             : ws(ws_)
             , decorator(decorator_)
         {
@@ -365,7 +367,8 @@ accept_ex(ResponseDecorator const& decorator, error_code& ec)
 
 template<class NextLayer>
 template<class ConstBufferSequence>
-void
+typename std::enable_if<! http::detail::is_header<
+    ConstBufferSequence>::value>::type
 stream<NextLayer>::
 accept(ConstBufferSequence const& buffers)
 {
@@ -383,7 +386,8 @@ accept(ConstBufferSequence const& buffers)
 template<class NextLayer>
 template<
     class ConstBufferSequence, class ResponseDecorator>
-void
+typename std::enable_if<! http::detail::is_header<
+    ConstBufferSequence>::value>::type
 stream<NextLayer>::
 accept_ex(ConstBufferSequence const& buffers,
     ResponseDecorator const &decorator)
@@ -404,7 +408,8 @@ accept_ex(ConstBufferSequence const& buffers,
 
 template<class NextLayer>
 template<class ConstBufferSequence>
-void
+typename std::enable_if<! http::detail::is_header<
+    ConstBufferSequence>::value>::type
 stream<NextLayer>::
 accept(ConstBufferSequence const& buffers, error_code& ec)
 {
@@ -425,7 +430,8 @@ accept(ConstBufferSequence const& buffers, error_code& ec)
 template<class NextLayer>
 template<
     class ConstBufferSequence, class ResponseDecorator>
-void
+typename std::enable_if<! http::detail::is_header<
+    ConstBufferSequence>::value>::type
 stream<NextLayer>::
 accept_ex(ConstBufferSequence const& buffers,
     ResponseDecorator const& decorator, error_code& ec)
@@ -641,8 +647,9 @@ async_accept_ex(ResponseDecorator const& decorator,
 
 template<class NextLayer>
 template<class ConstBufferSequence, class AcceptHandler>
-async_return_type<
-    AcceptHandler, void(error_code)>
+typename std::enable_if<
+    ! http::detail::is_header<ConstBufferSequence>::value,
+    async_return_type<AcceptHandler, void(error_code)>>::type
 stream<NextLayer>::
 async_accept(ConstBufferSequence const& buffers,
     AcceptHandler&& handler)
@@ -664,8 +671,9 @@ async_accept(ConstBufferSequence const& buffers,
 template<class NextLayer>
 template<class ConstBufferSequence,
     class ResponseDecorator, class AcceptHandler>
-async_return_type<
-    AcceptHandler, void(error_code)>
+typename std::enable_if<
+    ! http::detail::is_header<ConstBufferSequence>::value,
+    async_return_type<AcceptHandler, void(error_code)>>::type
 stream<NextLayer>::
 async_accept_ex(ConstBufferSequence const& buffers,
     ResponseDecorator const& decorator,

include/beast/websocket/impl/handshake.ipp

@@ -9,6 +9,7 @@
 #define BEAST_WEBSOCKET_IMPL_HANDSHAKE_IPP
 
 #include <beast/websocket/detail/type_traits.hpp>
+#include <beast/http/empty_body.hpp>
 #include <beast/http/message.hpp>
 #include <beast/http/read.hpp>
 #include <beast/http/write.hpp>
@@ -38,7 +39,8 @@ class stream<NextLayer>::handshake_op
         stream<NextLayer>& ws;
         response_type* res_p;
         detail::sec_ws_key_type key;
-        request_type req;
+        http::message<true,
+            http::empty_body, http::fields> req;
         response_type res;
         int state = 0;
 

include/beast/websocket/impl/stream.ipp

@@ -196,10 +196,10 @@ build_request(detail::sec_ws_key_type& key,
 }
 
 template<class NextLayer>
-template<class Decorator>
+template<class Fields, class Decorator>
 response_type
 stream<NextLayer>::
-build_response(request_type const& req,
+build_response(http::header<true, Fields> const& req,
     Decorator const& decorator)
 {
     auto const decorate =