Auto merge of #13281 - gilbertw1:basic-auth-cache-clean, r=jdm

Update basic auth cache to key off of origin instead of url

This pull request's primary purpose is to store basic auth credentials based on the url origin instead of the entire url. This fixes an issue where servo continuously prompts the user for credentials any time a basic auth secured resource is requested even though the user has already entered auth credentials for a different resource from the same origin.

The test associated with this PR hides image redirects behind a python handler that requires basic authentication. The reference page loads two images by directly specifying the image to load, while the test page loads the two images using the basic auth redirect handler with only the first image tag providing auth credentials.

I'd like to point a few specific items for review:
* url::Origin does not derive ```Hash```, so I am using ```ascii_serialization``` as the cache key. This seems like a stable enough representation.
* I've updated the http loader to store credentials not only on Success responses, but Redirect responses as well. I stumbled on this because nginx was redirecting 'test' -> 'test/' in my testing, and other browsers were storing the credentials on the redirect response vs. prompting for credentials a second time.
* In the test I'm using a timeout to load the second image (without authentication), otherwise the order that the images were loaded was unpredictable.

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- [x] These changes fix #12095  (github issue number if applicable).

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