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/fields.cpp

@@ -48,7 +48,7 @@ public:
             ::operator delete(p);
         }
 
-        template <class U>
+        template<class U>
         friend
         bool
         operator==(test_allocator<T> const&, test_allocator<U> const&) noexcept