HTTP authentication tokens are not removed anymore for third party requests

Fixing #13742 (moved) broke stripping off the Authorization header in case we are about to send it with third party requests. The relevant code in stanford-safecache.js was:

      try {
        // Disable 3rd party http auth, but exempt the browser (for favicon loads)
        // FIXME: Hrmm, this is just going to disable auth for 3rd party domains.
        // It would be better if we could isolate the auth, but still
        // allow it to be transmitted.. But I guess, who still uses http auth anyways?
        if (channel.getRequestHeader("Authorization") !== null) {
          if (parent_spec == "chrome://browser/content/browser.xul") {
            torbutton_log(3, "Allowing auth for browser load of "+channel.URI.spec);
          } else {
            torbutton_safelog(4, "Removing 3rd party HTTP auth for url ",
                              channel.URI.spec+", parent: "+parent_spec);
            channel.setRequestHeader("Authorization", null, false);
            channel.setRequestHeader("Pragma", null, false);
            channel.setRequestHeader("Cache-Control", null, false);
          }
        }
      } catch (e) {}

This is not an issue in our current stable but we need to get that fixed rather soon as we want to backport the fix for #13742 (moved) to our 4.0 series.