fix: websocket conns do not require Connection: upgrade header (#161)

It turns out that the WebSocket API provided by browsers [1]
does not actually send the `Connection: upgrade` header that our
websocket implementation (wrongly?) requires, so here we're dropping
that requirement.

[1]: https://developer.mozilla.org/en-US/docs/Web/API/WebSocket

httpbin/websocket/websocket.go

@@ -115,9 +115,6 @@ func (s *WebSocket) Handshake() error {
 		panic("websocket: handshake already completed")
 	}
 
-	if strings.ToLower(s.r.Header.Get("Connection")) != "upgrade" {
-		return fmt.Errorf("missing required `Connection: upgrade` header")
-	}
 	if strings.ToLower(s.r.Header.Get("Upgrade")) != "websocket" {
 		return fmt.Errorf("missing required `Upgrade: websocket` header")
 	}

httpbin/websocket/websocket_test.go

@@ -51,22 +51,22 @@ func TestHandshake(t *testing.T) {
 			},
 			wantStatus: http.StatusSwitchingProtocols,
 		},
-		"missing Connection header": {
+		"missing Connection header is okay": {
 			reqHeaders: map[string]string{
 				"Upgrade":               "websocket",
 				"Sec-WebSocket-Key":     "dGhlIHNhbXBsZSBub25jZQ==",
 				"Sec-WebSocket-Version": "13",
 			},
-			wantStatus: http.StatusBadRequest,
+			wantStatus: http.StatusSwitchingProtocols,
 		},
-		"incorrect Connection header": {
+		"incorrect Connection header is also okay": {
 			reqHeaders: map[string]string{
-				"Connection":            "close",
+				"Connection":            "foo",
 				"Upgrade":               "websocket",
 				"Sec-WebSocket-Key":     "dGhlIHNhbXBsZSBub25jZQ==",
 				"Sec-WebSocket-Version": "13",
 			},
-			wantStatus: http.StatusBadRequest,
+			wantStatus: http.StatusSwitchingProtocols,
 		},
 		"missing Upgrade header": {
 			reqHeaders: map[string]string{