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/span_body.cpp

@@ -34,7 +34,7 @@ struct span_body_test
             BEAST_EXPECT(req.body().size() == 3);
             BEAST_EXPECT(B::size(req.body()) == 3);
 
-            B::writer r{req};
+            B::writer r{req, req.body()};
             error_code ec;
             r.init(ec);
             BEAST_EXPECTS(! ec, ec.message());
@@ -50,7 +50,7 @@ struct span_body_test
             using B = span_body<char>;
             request<B> req;
             req.body() = span<char>{buf, sizeof(buf)};
-            B::reader w{req};
+            B::reader w{req, req.body()};
             error_code ec;
             w.init(boost::none, ec);
             BEAST_EXPECTS(! ec, ec.message());