rgw: Follow guidelines when verifying SigV4 signatures

According to Amazon's documentation[1], we must ensure that:
 - That `host` appears in CanonicalHeaders
 - If `content-type` is present, it appear in CanonicalHeaders
 - Any x-amz header appear in CanonicalHeaders.

[1] https://docs.aws.amazon.com/IAM/latest/UserGuide/reference_sigv-create-signed-request.html

Signed-off-by: Adam C. Emerson <aemerson@redhat.com>