cleanup: stop referencing depreceted HeaderMap field

Remove reference to the deprecated ResponseRecorder.HeaderMap field,
instead calling .Response.Header() to get the same data.

closes #10520

api/internal/testutil/responsewriter.go

@@ -50,7 +50,7 @@ func (r *ResponseRecorder) Header() http.Header {
 func (r *ResponseRecorder) HeaderMap() http.Header {
 	r.mu.Lock()
 	defer r.mu.Unlock()
-	return r.rr.HeaderMap
+	return r.rr.Result().Header
 }
 
 // Write to the underlying response buffer. Safe to call concurrent with Read.