Recommend a different Onion-Location config for Apache 2

Recommend the following type of construction at the [Onion-Location][] docs:

Header set Onion-Location "http://testkvehgz4pi23mj6wqhb2hkglx2omcelqjams4f3ph2gklqkxhikid.onion%{REQUEST_URI}s"

Instead of:

Header set Onion-Location expr="http://testkvehgz4pi23mj6wqhb2hkglx2omcelqjams4f3ph2gklqkxhikid.onion%{REQUEST_URI}"

References:

• mod proxy - Null Value for REQUEST_URI from Apache Web Server - Stack Overflow
• Expressions in Apache HTTP Server - Apache HTTP Server Version 2.4