Use relative path for Bing metadata

[slozier]

With the leading slash the url is always rooted. For example if I give the url http://foo.bar/foo/bar/ it will query http://foo.bar/REST/v1/Imagery/Metadata/... instead of the expected http://foo.bar/foo/bar/REST/v1/Imagery/Metadata/....

[cesium-concierge]

@slozier, thanks for the pull request! Maintainers, we have a signed CLA from @slozier, so you can review this at any time.

⚠️ I noticed that CHANGES.md has not been updated. If this change updates the public API in any way, fixes a bug, or makes any non-trivial update, please add a bullet point to CHANGES.md and comment on this pull request so we know it was updated. For more info, see the Pull Request Guidelines.

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[shunter]

Looks good to me. I added some tests, so I'll merge via command line.

I also made a change to add a slash to the provided URL. http://foo.bar/foo/bar/ and http://foo.bar/foo/bar should be treated the same.

Thanks for the fix. I assume you have a private Bing Maps server? (what used to be called "Virtual Earth Server")

[pjcozzi]

Thanks again @slozier!

@shunter if this warrants an update to CHANGES.md in master, can you please make the change.