http: Fix segfault when seeking on read

Previously, the dav backend would segfault when reading after a seek (or
also if you did a read_async() without an explicit send()/send_async()
first) because the stream from soup_request_send_finish() was not being
stored, so store it.

https://bugzilla.gnome.org/show_bug.cgi?id=710534
http: Ensure the range header is updated

Ensure that the range header is updated every time ensure_request() is
called in case it has been updated.

https://bugzilla.gnome.org/show_bug.cgi?id=710534
http: Fix the SEEK_END offset calculation

Fix the SEEK_END offset calculation by reversing the sign of offset and
taking into account the offset of the previous seek.

https://bugzilla.gnome.org/show_bug.cgi?id=710534
http: Allow seek past end of file

Handle a read after a seek past the end of the file by ignoring the
requested range not satisfiable http error (416) and simply returning 0.

https://bugzilla.gnome.org/show_bug.cgi?id=710534