如何在HttpRequest中设置用户代理（单元测试）

I've got a method that takes in an HttpRequest object and performs differently based on what User Agent is set. I'm trying to unit test it and am having a heck of a time tracking down how to set the User Agent string. I've tried the raw HttpRequest object and a variety of other objects that eventually yield it to no avail.I've got a method that takes in an HttpRequest