Add support for the `auto_merge` field in `PullRequest`

[neilmayhew]

The PullRequest type doesn't have a field for the auto_merge field in API responses. This field is documented in both the example response and the response schema.

[iphydf]

+1 for this feature request.

Also, I've been working on this: https://github.com/TokTok/hs-schema/blob/master/src/Data/Schema/OpenAPI/Haskell.hs which should someday be able to generate all the haskell types from the ghes API description yaml files (that should make these requests quite straightforward to resolve). Contributions there are welcome :) I have little time to work on it at the moment.