Setting the host in a functional test

Hi,
I have code that pulls out a google api key from the database depending
on the host. This allows me to have separate dev, test, and prod
domains.

I’m writing a test to check the key used, but in order to do so I need
to simulate the request is coming from live.myapp.com, test.myapp.com,
or localhost as the keys are keyed on the domain.

Does anyone know how to do this?

Thanks in advance,
John