[BUGFIX] Mirror selection is not respected

When a specific mirror is selected in Extension Manager,
the method used to return it fails.

Change-Id: Ibb60edec1a7fac4e3c217aad47d9332f7edd5fc6
Resolves: #27755
Releases: 4.6, 4.5
Reviewed-on: http://review.typo3.org/3999
Reviewed-by: Oliver Hader
Tested-by: Oliver Hader
Reviewed-by: Steffen Kamper
Tested-by: Steffen Kamper