* Compare final data instead of url
The behavior of did-finish-load and getURL has changed for redirects when
using NetworkService, so the test fails for NetworkService.
Comparing the finally received data makes the test more reliable.
* Implement intercept APIs
* Setting mimeType should set "content-type" header
* Passing no argument should not throw JS error
* Don't access api namespace in ProxyingURLLoaderFactory
* No need to create AtomURLLoaderFactory every time
* No use of weak factory
