Browser WS Connection: auto reconnect is not handling null values

[Felixduelmer]

In the browser ws connection, the auto-reconnect function is not handling null values.

If the onOpen function of the Browser WS Connection Websocket receives the correct data the completer finishes with a null value. This threw an error as the return function was non-nullable.

In the Server WS Connection, this is handled correctly.
Pull request is open: #18

[shamblett]

Thanks, package re published at version 3.1.3, please re test.