rc: fix core/command giving 500 internal error - fixes #4914

Before this change calling core/command gave the error

    error: response object is required expecting *http.ResponseWriter value for key "_response" (was *http.response)

This was because the http.ResponseWriter is an interface not an object.

Removing the `*` fixes the problem.

This also indicates that this bit of code wasn't properly tested.

fs/rc/params.go

@@ -112,15 +112,15 @@ func (p Params) GetHTTPRequest() (*http.Request, error) {
 //
 // If the parameter isn't found then error will be of type
 // ErrParamNotFound and the returned value will be nil.
-func (p Params) GetHTTPResponseWriter() (*http.ResponseWriter, error) {
+func (p Params) GetHTTPResponseWriter() (http.ResponseWriter, error) {
 	key := "_response"
 	value, err := p.Get(key)
 	if err != nil {
 		return nil, err
 	}
-	request, ok := value.(*http.ResponseWriter)
+	request, ok := value.(http.ResponseWriter)
 	if !ok {
-		return nil, ErrParamInvalid{errors.Errorf("expecting *http.ResponseWriter value for key %q (was %T)", key, value)}
+		return nil, ErrParamInvalid{errors.Errorf("expecting http.ResponseWriter value for key %q (was %T)", key, value)}
 	}
 	return request, nil
 }