Add unit tests for GitHub pagination

[joshsmith]

Problem

We need to add unit tests for the GitHub.API pagination that actually test that it paginates.

• Probably need to be able to specify per_page and status in the options
• Probably mock the returned headers
• Probably needs to be multiple fixtures for each page of the response
• Must test at least 3 pages to be able to get the full testing benefit here
• Should use the baxterthehacker/public-repo stub repository for response fixtures

Blocked by #1131