proxy: Fix #1574; health check now respects hostname when upstream Host header is configured (#1577)

* Implement adding Host header to health check

* Fix type problems

* Fix duplicate function, Replace args

* Add debugging

* Add debugging

* Add debugging

* Add debugging

* Attempt to set req.Host instead of the header

* Clean up debugging

* Fix missing newline

* Fix spelling

* Add test, refactoring

* Fix with gofmt

* Add error check on NewRequest