[BUGFIX] Reallow custom request context in getUrl

In b4813ddb an patch
has been introduced which respects the default stream context
when requesting urls. But the change prevents that custom
stream options (headers) can be set.

The default stream context should be used and extended by the
given custom headers.

Change-Id: I6ec6d23498e194055cefc1c6c63db7c161ee03c6
Resolves: #72907
Related: #70374
Releases: master, 7.6
Reviewed-on: https://review.typo3.org/46208


Reviewed-by: Stephan Großberndt <stephan@grossberndt.de>
Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: Frank Naegler <frank.naegler@typo3.org>
Reviewed-by: Claus Due <claus@phpmind.net>
Reviewed-by: Ralf Merz <info@merzilla.de>
Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>