[BUGFIX] Ignore cURL proxy header block

[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
(cherry picked from commit 09f1f5632e05a9140a7d65ceca0f38caa5f335eb)
Reviewed-on: https://review.typo3.org/47294
Reviewed-by: Jonas Götze <jonnsn@gmail.com>
Reviewed-by: Andreas Wolf <andreas.wolf@typo3.org>
Tested-by: Andreas Wolf <andreas.wolf@typo3.org>