[BUGFIX] Allow overriding of Backend Routes

Adding backend routes from Configuration/Backend/Routes.php is
done via "+=" operator which does not allow to override routes with
the same name. This is however a wanted behaviour - allowing extensions
to override core routes.

The patch changes the "+=" operator to a simple array_merge() call
to allow to change existing routes.

Resolves: #74004
Releases: master, 7.6
Change-Id: I7caf1ab5a91f849b4789ea352c20af6239470f2f
Reviewed-on: https://review.typo3.org/47005


Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Thomas Schlumberger <thomas@b13.de>
Tested-by: Thomas Schlumberger <thomas@b13.de>
Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: Frank Naegler <frank.naegler@typo3.org>