Fix EZP-23107: Custom headers break if header value contains semicolons

JIRA: https://jira.ez.no/browse/EZP-23107

# Description

This patch makes sure it's possible to configure eZ Publish to send a custom header value with a semicolon. This issue comes from the fact that the semicolon is a separator in the custom HTTP header settings (to optionally set the depth and level) and the case where the custom HTTP value would contain a semicolon was not taken into account.

# Tests

manual tests