Added tests that prove retryBucket.CreateObject is still buggy.
Added tests that prove retryBucket.CreateObject is still buggy.

The "fix" in commit 653da25 for jacobsa/comeback#18 didn't actually fix
the issue, because the bytes.Reader is not reset for each call. I'm
still seeing unexpectedly empty object issues in GCS:

    googleapi: Error 400: Provided CRC32C "tXUHAA==" doesn't match calculated CRC32C "AAAAAA==".

jacobsa/gcloud