The `ioutil.NopCloser(reader)` was hiding nested hash readers.
We make it an `io.Closer` so it can be attached without wrapping
and allows for nesting, by merging the requests.
- Use it to send the Content-MD5 header correctly encoded to S3
Gateway
- Fixes a bug in PutObject (including anonymous PutObject) and
PutObjectPart with S3 Gateway found when testing with Mint.
Verify() was being called by caller after the data
has been successfully read after io.EOF. This disconnection
opens a race under concurrent access to such an object.
Verification is not necessary outside of Read() call,
we can simply just do checksum verification right inside
Read() call at io.EOF.
This approach simplifies the usage.
