canThis() improvements

- Handle passing undefined user to canThis
  - Add existence check to parseContext if statement
  - Add unit test that passes undefined to canThis
- Allow internal canThis() checks
  - Allow passing 'internal' or { internal: true } as context
  - Do not lookup user permissions unless context.user found
  - If context.internal, resolve immediately
  - Add unit tests for passing 'internal' and { internal: true }
Merge pull request #2461 from jgable/canThisInternal