QNetworkAccessManager caches credentials per user@host,not per user@host/path

This causes the QtWebKit test http/tests/loading/basic-credentials-sent-automatically.html to fail.

The expected behaviour there is:

"This test makes sure that once an HTTP Basic Auth. protected path is authenticated once, urls that emanate from that path automatically have their credentials sent without a challenge.<br>
The first frame's path is /loading/resources/subresources/protected-resource.php, and we should get a challenge for it.<br>
It will be authorized with first/first-pw.<br>
The second frame's path is /loading/resources/protected-resource.php, and we should get a challenge for it, because it does not share a common subpath of the previously authorized resource.<br>
It will be authorized with second/second-pw.<br>
The third frame's path is also /loading/resources/protected-resource.php, and we should not get a challenge for it because we authorized to this path for the second frame.<br>
It will be authorized with second/second-pw.<br>
The fourth frame's path is /loading/resources/othersubresources/protected-resource.php, and we should not get a challenge for it, because it has a common subpath with the previously authorized second and third frames.<br>
It will be authorized with second/second-pw.<br>"