update method
Updates the value of a key. Fails if it does not exist.
Implementation
@override
Future<void> update(String key, Map<String, dynamic> value) async {
_checkInitialized();
final String keyWithPrefix = _addPrefix(key);
if (!map.containsKey(keyWithPrefix)) {
throw Errors.getInternalError(Errors.NO_MATCHING_KEY);
} else {
_map[keyWithPrefix] = value;
await _updatePref(keyWithPrefix, value);
}
}