SPBucket: Prevents empty lastChangeSignature scenarios

There are several flows that could, potentially, lead to multiple calls to `SPWebSocketChannel :: allVersionsFinishedForBucket` method.

If that happens, we could, potentially, end up with a SPBucket instance with a nil `lastChangeSignature field.

Fixes #325 

Can we be paranoid here and check pendingLastChangeSignature for nil before we set?