Don't handle redirects in HeadersReceived

There is a race condition in doing this, getting the headers can happen
so fast that the redirect and the original page requests are mixed up.

Delaying the redirect to DataReceived (if there is any data) or
RequestFinished (if there is none) avoids the issue and makes the code
simpler.

Fixes 311520.
Bump version to 1.4.8.

Just fix that annoying redirect problem.