polling: ensure that the timestamp used for cache busting is unique

This fixes https://github.com/LearnBoost/engine.io/issues/233.
The problem there was that the new request was being made with the same timestamp, generating a little request loop that lasted until a different timestamp was generated.

The same thing is done here https://github.com/LearnBoost/engine.io-client/blob/bab992d0f78d12de2696c21df0862da0714ab2f1/lib/transports/websocket.js#L194 but i don't know if the fix is required for the websocket transport, i think that it doesn't make sense in this case.