Publish major version for aws-sigv4

[steveh]

RE: #3027

It's probably too late now unless you yank the published version, but in my opinion the change to add content-type should have been a major version upgrade for aws-sigv4. This change was not backwards compatible with existing versions of aws-sdk-s3, and is not obvious from the changelog of either aws-sdk-s3 or aws-sigv4.

[mullermp]

Thank you for the feedback. Bumping the sigv4 major version would have caused more conflict because of how it's depended on for aws-sdk-core and the service gems, and common usage of more than one gem, would have caused a conflict of two major versions in bundle resolution.