Fixes, fail testing:

Core:

* Test buffer_cat iterator move members

HTTP:

* Fixed yield / resume in writer
* Fixed message serialization with chunked encoding

* Test yield / resume in writer
* Test all conditional branches during message serialization
* Test chunked encoding
* Increase coverage on parse_error
* Add parse_error::general

WebSocket:

* Add error::general
* Increase coverage in error

test/websocket/detail/frame.cpp

@@ -154,6 +154,14 @@ public:
             check(fh);
             fh.rsv1 = false;
 
+            fh.rsv2 = true;
+            check(fh);
+            fh.rsv2 = false;
+
+            fh.rsv3 = true;
+            check(fh);
+            fh.rsv3 = false;
+
             fh.op = opcode::rsv3;
             check(fh);
             fh.op = opcode::text;