Refactor method and verb (API Change):

The verb interfaces now use verb::unknown instead of
boost::optional<verb> == boost::none to indicate that
the request-method is an unrecognized string.

The http::header interface is modified to focus more on the
verb enum rather than the string. For recognized verbs, the
implementation stores an integer instead of the string.
Unknown verbs are still stored as strings.

* header::method now returns a verb
* header::method_string returns the method text

include/beast/websocket/impl/rfc6455.ipp

@@ -19,7 +19,7 @@ is_upgrade(http::header<true, Fields> const& req)
 {
     if(req.version < 11)
         return false;
-    if(req.method() != "GET")
+    if(req.method() != http::verb::get)
         return false;
     if(! http::is_upgrade(req))
         return false;

include/beast/websocket/impl/stream.ipp

@@ -225,7 +225,7 @@ build_response(http::header<true, Fields> const& req,
         };
     if(req.version < 11)
         return err("HTTP version 1.1 required");
-    if(req.method() != "GET")
+    if(req.method() != http::verb::get)
         return err("Wrong method");
     if(! is_upgrade(req))
         return err("Expected Upgrade request");