immutable repairer: fix DownUpConnector so that it satisfies short reads the were...

immutable repairer: fix DownUpConnector so that it satisfies short reads the were requested after the last write and before the close
This is probably the cause of the very rare "loss of progress" bug.  This is tested by unit tests.  A recent patch changed this to errback instead of losing progress, and now this patch is changing it again to return a short read instead of errbacking.  Returning a short read is what the uploader (in encode.py) is expecting, when it is reading the last block of the ciphertext, which might be shorter than the other blocks.