Don't rewrite Bing tile URLs to https, instead just ask for https

[kring]

For ~5 years now, we've had code in BingMapsImageryProvider that rewrote the tile URL from http:// to https:// if the page was on an https connection. This seemed kind of dodgy, but we thought it was necessary to avoid mixed content warnings.

But, it turns out that Bing Maps (now?) has a way to directly ask for an https tile URL, by adding &uriScheme=https to the metadata URL. And it also turns out that this is totally necessary with the new "on demand" styles added in #7808 because, if we just rewrite the URL as we were doing before, we get invalid certificate errors. Bing Maps returns an akamai.net certificate when we're connecting on a virtualearth.net domain.

So this PR switches to using the uriScheme mechanism. The new on-demand styles won't work on https pages without this PR (ask me how I know).

[cesium-concierge]

Thanks for the pull request @kring!

• ✔️ 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.

[kring]

By the way, I've arranged the logic so that we use useScheme=https for all page protocols except http, which uses uriScheme=http. Specifically file:// URLs and the like will now use https where previously they used http. Arguably we could just use https everywhere, but Bing hasn't changed the default so maybe we shouldn't either I guess.

[kring]

I haven't updated CHANGES.md because this only affects the on-demand styles which haven't been released yet.

[mramato]

Any reason not to just always use https these days? Otherwise looks good to me. @shunter any thoughts here?

[shunter]

Bing doesn't use HTTP/2 yet, so HTTPS will have more overhead than HTTP, so that seems like keeping the current default is fine (plus as Kevin says HTTP is Bing's default too)

This works as expected over both HTTP and HTTPS. Merging.