[BUGFIX] Ignore cURL proxy header block

When cURL is enabled with a proxy, the proxy’s HTTP header (sent as a
response to the client CONNECT request) was not removed correctly for
https requests.

See also RFC 2817.

Resolves: #73567
Releases: master, 7.6, 6.2
Change-Id: I0f11933f523b099dd23a5bef631699904ffcefc8
Reviewed-on: https://review.typo3.org/46779


Reviewed-by: Andreas Wolf <andreas.wolf@typo3.org>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>
Tested-by: Andreas Wolf <andreas.wolf@typo3.org>