Add optional 'author' param when making commits

What does this MR do?

This fulfills https://gitlab.com/gitlab-org/gitlab-ce/issues/20789 by adding the ability to specify a user's ID to be used as the author of a commit.

Why was this MR needed?

To fulfill https://gitlab.com/gitlab-org/gitlab-ce/issues/20789

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/20789

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
  • Will do after this has been reviewed
• Documentation created/updated
  • Updated the File API doc
• API support added
  • This MR adds an optional param to the Files API requests
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together