Setting `override_host` in a backend definition doesn't always create `.host_header` in the backend's VCL definition.

[bcarrier92]

In our Fastly terraform configuration, I added override_host to one of my backends (let’s called it backend-a). When I deployed this change, I expected to see .host_header = <my value>; under backend-a’s definition in the VCL in the Fastly console, but I did not.