Fixes#1707
The `Expect` HTTP header is ignored by the AWS SDK SigV4 signer and is omitted during signature calculation. As a result, the signature is computed incorrectly when the `Expect` header is included in the signed headers. This PR removes the `Expect` header from the SigV4 ignored headers list in the SDK-derived source code.
This fixes the case where clients can include the content-length
header in the signed headers for a 0 length file (like s3cmd).
Since we had to hoist the aws code into versitygw, we can also
remove the hack for the "User-Agent" header in the hard coded
excludes list and just remove it from the excludes list.
