Refactor http::header contents (API Change):

fix #124

The http::header data members "method", "url", and "reason"
are changed from data members, to pairs of get and set
functions which forward the call to the Fields type used
to instantiate the template.

Previously, these data members were implemented using
std::string. With this change, the implementation of the
Fields type used to instantiate the template is now in
control of the representation of those values. This permits
custom memory allocation strategies including uniform use of
the Allocator type already provided to beast::http::basic_fields.

include/beast/websocket/impl/stream.ipp

@@ -128,7 +128,7 @@ void
 stream<NextLayer>::
 do_handshake(response_type* res_p,
     boost::string_ref const& host,
-        boost::string_ref const& resource,
+        boost::string_ref const& target,
             RequestDecorator const& decorator,
                 error_code& ec)
 {
@@ -137,7 +137,7 @@ do_handshake(response_type* res_p,
     detail::sec_ws_key_type key;
     {
         auto const req = build_request(
-            key, host, resource, decorator);
+            key, host, target, decorator);
         pmd_read(pmd_config_, req.fields);
         http::write(stream_, req, ec);
     }
@@ -157,13 +157,13 @@ request_type
 stream<NextLayer>::
 build_request(detail::sec_ws_key_type& key,
     boost::string_ref const& host,
-        boost::string_ref const& resource,
+        boost::string_ref const& target,
             Decorator const& decorator)
 {
     request_type req;
-    req.url = { resource.data(), resource.size() };
+    req.target(target);
     req.version = 11;
-    req.method = "GET";
+    req.method("GET");
     req.fields.insert("Host", host);
     req.fields.insert("Upgrade", "websocket");
     req.fields.insert("Connection", "upgrade");
@@ -215,7 +215,7 @@ build_response(request_type const& req,
         {
             response_type res;
             res.status = 400;
-            res.reason = http::reason_string(res.status);
+            res.reason(http::reason_string(res.status));
             res.version = req.version;
             res.body = text;
             prepare(res);
@@ -224,7 +224,7 @@ build_response(request_type const& req,
         };
     if(req.version < 11)
         return err("HTTP version 1.1 required");
-    if(req.method != "GET")
+    if(req.method() != "GET")
         return err("Wrong method");
     if(! is_upgrade(req))
         return err("Expected Upgrade request");
@@ -246,7 +246,7 @@ build_response(request_type const& req,
         {
             response_type res;
             res.status = 426;
-            res.reason = http::reason_string(res.status);
+            res.reason(http::reason_string(res.status));
             res.version = req.version;
             res.fields.insert("Sec-WebSocket-Version", "13");
             prepare(res);
@@ -264,7 +264,7 @@ build_response(request_type const& req,
             res.fields, unused, offer, pmd_opts_);
     }
     res.status = 101;
-    res.reason = http::reason_string(res.status);
+    res.reason(http::reason_string(res.status));
     res.version = req.version;
     res.fields.insert("Upgrade", "websocket");
     res.fields.insert("Connection", "upgrade");