PullRequest struct is missing "rebaseable" field

[nghialv]

API: https://developer.github.com/v3/pulls/#get-a-single-pull-request

https://godoc.org/github.com/google/go-github/github#PullRequest

[gmlewis]

Thank you, @nghialv!

This would be a great PR for any new contributor to this repo or a new Go developer.
All contributions are greatly appreciated!

Feel free to volunteer for any issue and the issue can be assigned to you so that others don't attempt to duplicate the work.

Please check out our CONTRIBUTING.md guide to get started.

Thank you!

[cpanato]

@gmlewis can i take this one?

[gmlewis]

Yes, @cpanato - it is yours. Thank you!

[gmlewis]

Oh, and while you are looking at the PullRequest struct, if you could please check for other missing "normal" fields (other than the *_url ones, as we only add those on demand), that would be great. Thanks!

[cpanato]

sure @gmlewis

[nghialv]

Nice. Thank you.