Add ability to add or remove access for beta groups to a build

[zormandi]

The PR

• adds a new endpoint for adding access for beta groups to a build, and
• fixes the endpoint that's used for removing access from beta groups to a build.

This is a slightly more involved change unfortunately, but it's necessary for these endpoints to work. Both endpoints expect the data parameter to be an Array and since this has always been an object (Hash) so far, there was no facility to pass in an Array parameter to the request. Also, the DELETE endpoint expects a body which is not allowed in Net::HTTP::Delete, so a custom DELETE class had to be used.

The DELETE endpoint was already implemented but I don't see how it could have ever worked since no request body could have been passed into it. It's working now but its alias changed to reflect the endpoint - this is technically a breaking change but as I said, I don't think this was working before.

I documented the usage in the README.

[kyledecot]

Wow–this is definitely a substantial update! Thanks for all of your hard work on this. Prior to this change only simply DELETE APIs were supported (such as deleting a bundle ID which only needs the ID in the URL path). Now we can support a lot more APIs 🎉