Imagery Provider Resource Fix

[ProjectBarks]

This solves an issue where createOpenStreetMapImageryProvider would improperly handle a Resource object.

I also fixed up MapboxImageryProvider to work in a more standardized and expected way.

This is related to #4812

[cesium-concierge]

Thanks for the pull request @ProjectBarks!

• ✔️ Signed CLA found.
• ❔ CHANGES.md was not updated.
  • If this change updates the public API in any way, please add a bullet point to CHANGES.md.
• ❔ Unit tests were not updated.
  • Make sure you've updated tests to reflect your changes, added tests for any new code, and ran the code coverage tool.

Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.
• Works (or fails gracefully) in IE11.

[mramato]

Looks like you have some test failures

[ProjectBarks]

@mramato Good catch. Submitted relevant changes.

[mramato]

Thanks!

[thw0rted]

I know this is already merged, but the current changes forgot to update the relevant documentation. Is it all generated from the JSDoc blocks?

[OmarShehata]

@thw0rted the docs are indeed generated from the JSDoc blocks. You can run npm run generateDocumentation to see the docs locally.