This time around instead of changing the protocol-relative URLs
to be matching the protocol of what Mahara is using I've changed
how html purifier checks iframes to allow protocol-relative URLs.

Change-Id: I2a9436ecf3f6046acdefce8ac7751c12ad2bbf9d
Signed-off-by: Robert Lyon <robertl@catalyst.net.nz>
Signed-off-by: Aaron Wells <aaronw@catalyst.net.nz>