[BUGFIX] RTE: Compressor fails on files with absolute url in FE

Solution: Use relative url.

Change-Id: I5bca5cd8567f9489e07ec928b9b2e046e970f212
Resolves: #30533
Releases: 4.6
Reviewed-on: http://review.typo3.org/5510
Reviewed-by: Stanislas Rolland
Tested-by: Stanislas Rolland