added safety to setUserId (when called twice)

README.md

@@ -144,3 +144,4 @@ Yjs is licensed under the [MIT License](./LICENSE.txt).
 
 [ShareJs]: https://github.com/share/ShareJS
 [OpenCoweb]: https://github.com/opencoweb/coweb/wiki
+

src/Connector.js

@@ -63,8 +63,12 @@ module.exports = function (Y/* :any */) {
       return this.y.db.stopGarbageCollector()
     }
     setUserId (userId) {
-      this.userId = userId
-      return this.y.db.setUserId(userId)
+      if (this.userId == null) {
+        this.userId = userId
+        return this.y.db.setUserId(userId)
+      } else {
+        return null
+      }
     }
     onUserEvent (f) {
       this.userEventListeners.push(f)