New BodyReader and BodyWriter constructors (API Change):

fix #884

* BodyReader and BodyWriter constructors now require the header and
  body elements to be passed as distinct header and value_type objects.

This enables the composition of body types, for example:

    http::response<compressed_body<http::string_body>> res;

* The previous single-argument constructors are deprecated and will be
  removed in a subsequent version.

Actions Required:

* Change user-defined instances of BodyReader or BodyWriter constructor
  signatures to the two-argument form.

OR

* Define the macro BOOST_BEAST_ALLOW_DEPRECATED in the project (which
  will accept both the new and the deprecated signatures).

test/beast/http/write.cpp

@@ -48,9 +48,8 @@ public:
 
             template<bool isRequest, class Fields>
             explicit
-            writer(message<isRequest,
-                    unsized_body, Fields> const& msg)
-                : body_(msg.body())
+            writer(header<isRequest, Fields> const&, value_type const& b)
+                : body_(b)
             {
             }
 
@@ -93,9 +92,8 @@ public:
 
             template<bool isRequest, class Fields>
             explicit
-            writer(message<isRequest,
-                    test_body, Fields> const& msg)
-                : body_(msg.body())
+            writer(header<isRequest, Fields> const&, value_type const& b)
+                : body_(b)
             {
             }
 
@@ -230,9 +228,8 @@ public:
 
             template<bool isRequest, class Fields>
             explicit
-            writer(message<isRequest,
-                    fail_body, Fields> const& msg)
-                : body_(msg.body())
+            writer(header<isRequest, Fields> const&, value_type const& b)
+                : body_(b)
             {
             }