rds_instance - fix idempotency when using non-lowercase for preferred…

…_maintenance_window (ansible-collections#516)

* rds_instance - preferred_maintenance_window - force day of week into lowercase
* add integration test
* changelog

This commit was initially merged in https://github.com/ansible-collections/community.aws
See: ansible-collections/community.aws@8fbfead

plugins/modules/rds_instance.py

@@ -1214,6 +1214,10 @@ def main():
     if module.params['processor_features'] is not None:
         module.params['processor_features'] = dict((k, to_text(v)) for k, v in module.params['processor_features'].items())
 
+    # Ensure dates are in lowercase
+    if module.params['preferred_maintenance_window']:
+        module.params['preferred_maintenance_window'] = module.params['preferred_maintenance_window'].lower()
+
     client = module.client('rds')
     changed = False
     state = module.params['state']