plugin/forward: Fix issue in DoH health checks used a default TLS instead of the configured CA (#8279)

This PR fixes issue in forward plugin where default TLS instead of
configured CA was used.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

plugin/forward/setup.go

@@ -244,6 +244,7 @@ func parseStanza(c *caddy.Controller) (*Forward, error) {
 				Timeout:   2 * time.Second,
 			}
 			f.proxies[i].SetHTTPClient(&c)
+			f.proxies[i].SetTLSConfig(f.tlsConfig)
 			f.proxies[i].SetDOHRequestOptions(f.dohMethod)
 		}