Copy auth field objects when serializing values

This fixes a bug where the password field would be reset to an empty
value when serializing the mutation values. Making a copy of the auth
field values ensures that we are not inadvertently mutating the form
object.

Changelog: fixed
EE: true

ee/app/assets/javascripts/security_configuration/dast_profiles/dast_site_profiles/components/dast_site_profile_form.vue

@@ -115,7 +115,7 @@ export default {
         .map((url) => url.trim());
     },
     serializedAuthFields() {
-      const authFields = this.authSection.fields;
+      const authFields = { ...this.authSection.fields };
       // not to send password value if unchanged
       if (authFields.password === REDACTED_PASSWORD) {
         delete authFields.password;