If the source supplied fewer bytes than its declared size, the upload
request failed but a retry could report success even though the stored
file was truncated, because the retry re-sent an already exhausted
reader.
Count the bytes actually read from the source and if they do not match
the declared size, remove the partially uploaded file and return an
error.
This was found by the new FsPutShortEOF integration test.
Nick Craig-Wood
5d057549b9