Camelize every body unless otherwise specified

Similar to the params, almost every BODY sent to YouTube API endpoints
expects camel-case attributes.

Of course, "almost" means not all of them; for instance authenticating
with YouTube requires to submit "grant_type" and not "grantType" in the body.
Use underscore, rather than camel-case body

Because of the previous commits, body attributes can be either underscore or
camel-case and Request will transform them in the format that is
appropriate for YouTube API endpoint.

For coherence with the rest of the code, body attributes are now spelled
out as underscore.
Add `content_owner.create_claim` and claim.delete