merge method
Implementation
void merge(Map<String, dynamic> values) {
// Existing keys are overwritten. The body is the source of truth for
// POST/PUT/PATCH, so the values land there; the merged view is
// patched too, so query-string keys of the same name are overridden
// as well.
body.addAll(values);
(_allData ??= <String, dynamic>{
...body,
..._query,
...params(),
}).addAll(values);
}