Override host backend setting can't be unset

[aabumuslimov]

Steps to reproduce:

1. Create new backend at Backend Settings area.
2. Edit new backend.
3. In advanced options set some value to the "Override host" field.
4. Save backend.
5. Edit the backend again and remove value from the "Override host" field.
6. Save backend.

Actual result:
After the backend is opened for editing again, "Override host" value stays. It is impossible to unset it using the module.
Field can be removed in the Fastly UI though.

[smaeda-ks]

Thanks for the report. I'll look into this