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/message.cpp

@@ -168,13 +168,13 @@ public:
     void testHeaders()
     {
         {
-            using req_type = request_headers<headers>;
+            using req_type = request_headers;
             static_assert(std::is_copy_constructible<req_type>::value, "");
             static_assert(std::is_move_constructible<req_type>::value, "");
             static_assert(std::is_copy_assignable<req_type>::value, "");
             static_assert(std::is_move_assignable<req_type>::value, "");
 
-            using res_type = response_headers<headers>;
+            using res_type = response_headers;
             static_assert(std::is_copy_constructible<res_type>::value, "");
             static_assert(std::is_move_constructible<res_type>::value, "");
             static_assert(std::is_copy_assignable<res_type>::value, "");
@@ -183,7 +183,7 @@ public:
 
         {
             MoveHeaders h;
-            request_headers<MoveHeaders> r{std::move(h)};
+            message_headers<true, MoveHeaders> r{std::move(h)};
             BEAST_EXPECT(h.moved_from);
             BEAST_EXPECT(r.headers.moved_to);
             request<string_body, MoveHeaders> m{std::move(r)};

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

test/http/parser_v1.cpp

@@ -64,7 +64,7 @@ public:
         streambuf rb;
         headers_parser_v1<true, headers> p0;
         parse(ss, rb, p0);
-        request_headers<headers> const& reqh = p0.get();
+        request_headers const& reqh = p0.get();
         BEAST_EXPECT(reqh.method == "GET");
         BEAST_EXPECT(reqh.url == "/");
         BEAST_EXPECT(reqh.version == 11);