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/serializer.cpp

@@ -20,6 +20,40 @@ namespace http {
 class serializer_test : public beast::unit_test::suite
 {
 public:
+    struct deprecated_body
+    {
+        using value_type = std::string;
+
+        class writer
+        {
+        public:
+            using const_buffers_type =
+                boost::asio::const_buffer;
+
+            value_type const& body_;
+
+            template<bool isRequest, class Fields>
+            explicit
+            writer(message<isRequest, deprecated_body, Fields> const& m):
+                body_{m.body()}
+            {
+            }
+
+            void init(error_code& ec)
+            {
+                ec.assign(0,  ec.category());
+            }
+
+            boost::optional<std::pair<const_buffers_type, bool>>
+            get(error_code& ec)
+            {
+                ec.assign(0, ec.category());
+                return {{const_buffers_type{
+                    body_.data(), body_.size()}, false}};
+            }
+        };
+    };
+
     struct const_body
     {
         struct value_type{};
@@ -30,7 +64,7 @@ public:
                 boost::asio::const_buffer;
 
             template<bool isRequest, class Fields>
-            writer(message<isRequest, const_body, Fields> const&);
+            writer(header<isRequest, Fields> const&, value_type const&);
 
             void
             init(error_code& ec);
@@ -50,7 +84,7 @@ public:
                 boost::asio::const_buffer;
 
             template<bool isRequest, class Fields>
-            writer(message<isRequest, mutable_body, Fields>&);
+            writer(header<isRequest, Fields>&, value_type&);
 
             void
             init(error_code& ec);
@@ -115,10 +149,20 @@ public:
         }
     }
 
+    void testBodyWriterCtor()
+    {
+        response<deprecated_body> res;
+        request<deprecated_body> req;
+        serializer<false, deprecated_body> sr1{res};
+        serializer<true, deprecated_body> sr2{req};
+        boost::ignore_unused(sr1, sr2);
+    }
+
     void
     run() override
     {
         testWriteLimit();
+        testBodyWriterCtor();
     }
 };