HTTP probe: For "Host" header set request's Host field.
HTTP probe: For "Host" header set request's Host field.

Fixes: https://github.com/google/cloudprober/issues/331
Background: https://github.com/golang/go/issues/7682
PiperOrigin-RevId: 285370151

google/cloudprober,google/cloudprober,cloudprober/cloudprober,cloudprober/cloudprober,cloudprober/cloudprober