Retrying a stream of PartialResultSets must correctly reset to the last state that it
had when it received a PartialResultSet with a resume token. That means:
1. The pending value that should be merged with the next result should be reset to the
   pending value of the last PartialResultSet with a resume token.
2. The stream must correctly check for the existence of a resume token.
3. The stream must ensure that all values that are already in the pipeline must be
   handled before resetting the stream to the last resume token to ensure that no
   PartialResultSet is added twice to the stream.

Fixes googleapis#1392

… issue-1392