Refactor to use httputil.ReverseProxy (#342)

* Refactor to use httputil.ReverseProxy

Refactor manual HTTP proxying logic in Process.ProxyRequest to use the standard
library's httputil.ReverseProxy.

* Refactor TestProcess_ForceStopWithKill test

Update to handle behavior with httputil.ReverseProxy.

* Fix gin interface conversion panic

proxy/process_test.go

@@ -436,7 +436,9 @@ func TestProcess_ForceStopWithKill(t *testing.T) {
 		if runtime.GOOS == "windows" {
 			assert.Contains(t, w.Body.String(), "wsarecv: An existing connection was forcibly closed by the remote host")
 		} else {
-			assert.Contains(t, w.Body.String(), "unexpected EOF")
+			// Upstream may be killed mid-response.
+			// Assert an incomplete or partial response.
+			assert.NotEqual(t, "12345", w.Body.String())
 		}
 
 		close(waitChan)