Before this fix we were reading the hash from the upload using the
string "ETag", however the go runtime normalises the tag into "Etag"
so we were in fact always reading an empty string.
This bug was introduced in
aeea4430d5fe8896 swift: efficiency: slim Object and reduce requests on upload
It was spotted by the integration tests.
The fix was just to use the canonical form "Etag" instead of "ETag".
9cc17cec9a