Header Modifier doesn't work for "User-Agent" property

[mniezgodka]

Header Modifier doesn't work for "User-Agent" property.

To reproduce:

1. Execute below suite
   suite.zip

Expected:
"User-Agent" property is being override with value: "Mozilla/5.0 (Windows NT 6.1; WOW64; rv:33.0) Gecko/20120101 Firefox/33.0"
and browser is recognized as: "You’re using Firefox 33."

Actual
"User-Agent" property isn't override and browser is recognized as: "You’re using Chrome 75."