Deleting collaborators with email addresses that include a plus.

- When using the actual API, trying to delete a collaborator with an
  email format of email+token@example.com returns a 404.

```
Heroku::API::Errors::NotFound: Expected(200) <=> Actual(404 Not Found)
```

I was able to solve this in my application by replacing `+` with `%2B` before making the call to `delete_collaborator`.

I added a test for deleting a collaborator, but this test currently passes. This problem doesn't seem like its too big of an issue, but I'm just curious as to what you guys think. I haven't dove into heroku.rb to think about a solution yet.