If the source supplied fewer bytes than its declared size, the single
part upload path accepted the short body and stored a truncated file
recorded with the declared size, reporting a successful upload. The
multipart path already checks for this.
Count the bytes actually read and fail the upload if they do not match
the declared size, which cancels the partially created file.
This was found by the FsPutShortEOF integration test.
Nick Craig-Wood
8e744de5e6