Explicitly require Host header in handshake

Per RFC 6455, 4.1 Client Requirements:

   4.   The request MUST contain a |Host| header field whose value
        contains /host/ plus optionally ":" followed by /port/ (when not
        using the default port).

The h11 library should already be verifying this, but it doesn't hurt to
repeat, and also assures mypy that it is not None.

Author: bluetech

Diff for: wsproto/handshake.py

@@ -169,7 +169,7 @@ def events(self) -> Generator[Event, None, None]:
 
     ############ Server mode methods
 
-    def _process_connection_request(self, event: h11.Request) -> Request:
+    def _process_connection_request(self, event: h11.Request) -> Request:  # noqa: MC0001
         if event.method != b"GET":
             raise RemoteProtocolError(
                 "Request method must be GET", event_hint=RejectConnection()
@@ -228,6 +228,10 @@ def _process_connection_request(self, event: h11.Request) -> Request:
             raise RemoteProtocolError(
                 "Missing header, 'Sec-WebSocket-Version'", event_hint=RejectConnection()
             )
+        if host is None:
+            raise RemoteProtocolError(
+                "Missing header, 'Host'", event_hint=RejectConnection()
+            )
 
         self._initiating_request = Request(
             extensions=extensions,