When we introduced LengthWrapper, we neglected to make it resettable. As
a result, upload failures result in errors like:
put_object(...) failure and no ability to reset contents for reupload.
Now, LengthWrappers will be resettable if their _readable has seek/tell.
Related-Change: I6c8bc1366dfb591a26d934a30cd21c9e6b9a04ce
Change-Id: I21f43f06e8c78b24d1fc081efedf2687942e042f