BadRequest exception when adding a collaborator

The following:

``` python
from pygithub3 import Github
Github(token='token').repos.collaborators.add('collaborator', user='user', repo='repo')
```

fails with `pygithub3.exceptions.BadRequest: 400 - Problems parsing JSON` after GitHub tries to interpret the `PLACEHOLDER` (submitted by `pygithub.services.base.Service._put`) as JSON.
