Refactor message and message_headers declarations:

message_headers is now a set of partial class template
specializations instead of a template class alias. This solves
a problem where template functions taking message_headers as a
parameter could not deduce argument types, since std::conditional
obscured the deduced context.

Both classes are refactored to share declarations using an #ifdef,
to eliminate an ugly set of extra declarations needed when building
the documentation.

Copy and move class special members are added.

A new function message::base() is provided which returns the
message_headers portion of a message.

test/http/nodejs_parser.hpp

@@ -818,7 +818,8 @@ private:
     {
         return on_request(method, url,
             major, minor, keep_alive, upgrade,
-                typename message_type::is_request{});
+                std::integral_constant<
+                    bool,  message_type::is_request>{});
     }
 
     bool
@@ -847,7 +848,7 @@ private:
     {
         return on_response(
             status, reason, major, minor, keep_alive, upgrade,
-                std::integral_constant<bool, ! message_type::is_request::value>{});
+                std::integral_constant<bool, ! message_type::is_request>{});
     }
 
     void