Return first position when interpolating rhumblines at surface distance 0.0

[likangning93]

In some cases, very small rhumblines can return an unexpected value when requesting a position at the beginning of the rhumbline (surface distance 0.0) using interpolateUsingSurfaceDistance.
This PR ensures that rhumbline interpolation always returns the start position when using surface distance 0.0.

[cesium-concierge]

Thanks for the pull request @likangning93!

• ✔️ 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.

Reviewers, don't forget to make sure that:

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

[lilleyse]

Just update CHANGES.md and this will be good to go. I can confirm the fix works on the problematic dataset.