Add support for Unit response type in HEAD method requests.

[forrestpangborn]

This adds support for Unit (additionally to preexisting Void support) response types for HEAD method requests.

[JakeWharton]

This will throw NoClassDefFound if you do not have the Kotlin stdlib as a runtime dep. The conditional needs to account for this either with a try/catch or a boolean with separate lookup mechanism.

[forrestpangborn]

I see a similar case elsewhere in the project doing this here. Would you recommend replicating this or attempting to extract into something reusable?

[forrestpangborn]

I added a static function Utils.isUnitAvailable() - will gladly move elsewhere or can convert to a local try/catch if thats preferable.

[JakeWharton]

Also needs tests!

[forrestpangborn]

@JakeWharton is there anything else you'd recommend or prefer for this?

I'm willing to change the approach it uses for determining if Unit is available at runtime. I can also add more tests if you think thats appropriate and if you can suggest what would also be good to verify.