Fix redirect blocking (http-on-modify-request notification fires delayed starting with Firefox 18)

Redirect handling is complicated, we can only see the HTTP channel there but not the additional data available in a content policy call. This means that we need to watch channel creation and to attach data from the preceding content policy call. The way to do that apparently changed in Firefox 18 (https://bugzilla.mozilla.org/show_bug.cgi?id=800799).

[Felix Dahlke, 2013-09-13 10:12:00]

LGTM