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.

test/websocket/stream.cpp

@@ -130,7 +130,8 @@ public:
         }
 
         template<class NextLayer, class Buffers>
-        void
+        typename std::enable_if<
+            ! http::detail::is_header<Buffers>::value>::type
         accept(stream<NextLayer>& ws,
             Buffers const& buffers) const
         {
@@ -165,7 +166,8 @@ public:
 
         template<class NextLayer,
             class Buffers, class Decorator>
-        void
+        typename std::enable_if<
+            ! http::detail::is_header<Buffers>::value>::type
         accept_ex(stream<NextLayer>& ws,
             Buffers const& buffers,
                 Decorator const& d) const
@@ -319,7 +321,8 @@ public:
         }
 
         template<class NextLayer, class Buffers>
-        void
+        typename std::enable_if<
+            ! http::detail::is_header<Buffers>::value>::type
         accept(stream<NextLayer>& ws,
             Buffers const& buffers) const
         {
@@ -367,7 +370,8 @@ public:
 
         template<class NextLayer,
             class Buffers, class Decorator>
-        void
+        typename std::enable_if<
+            ! http::detail::is_header<Buffers>::value>::type
         accept_ex(stream<NextLayer>& ws,
             Buffers const& buffers,
                 Decorator const& d) const