Allow nil error parameter in deleteDocumentFromRevision:error:
Increment revID generation when resurrecting documents

This fixes two bugs:

1. push replication failing when resurrecting docs.
2. resurrecting with identical data in both first generations failing.

test_CreateRemoteDeleteRemotePullPushCreateLocalPush reproduces the
scenario in the bug report for (1).

test_PushResurectedDocument is a minimal reproduction, which also
showed bug (2).