Fix md5 for gzip'ed content

MD5 is passed as value.md5, but this isn't valid for gzip'ed files and triggers the following error message.

```
<?xml version="1.0" encoding="UTF-8"?>
<Error><Code>BadDigest</Code><Message>The Content-MD5 you specified did not match what we received.</Message><ExpectedDigest>cBv2KZVF7F5fVeUbkkQGew==</ExpectedDigest><CalculatedDigest>o4oyfv3UJ+UmfOJ3kCa39w==</CalculatedDigest><RequestId>D458DEA2087B110E</RequestId><HostId>snip</HostId></Error>
```

The MD5 must be updated when using the gzip option rather than using the non-gzip'ed checksum.