Auto merge of servo#6722 - samfoo:sts-headers, r=jdm

Obey Strict-Transport-Security header

Resolves servo#6703.

Done:

* [x] When STS headers received, add the host to the HSTS list

Todo:

* [ ] Persist the in-memory list so that it's reloaded on the next browser boot
* [ ] Add tests to `http_loader::load` - it's pretty well completely untested right now, but it's a bit gnarly to untangle and without mocking, it's hard to deal with the dependency on making a real network request. Writing a mock request object should be doable for testing, but there's a lot going on in the function right now.

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