Don't send ion access token to non-ion servers

[mramato]

@tfili can you review. @OmarShehata this should fix the issue you uncovered.

Some ion-hosted assets can still point to additional external servers as, part of their dataset. We were still appending the ion access_token in the Accept header to these servers, which is at best pointless but at worst would cause some CORS requests to fail because the Accept header was not allowed.

[cesium-concierge]

Thanks for the pull request @mramato!

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