Fix hybrid sync & async requests mode

Respect explicit return_async_id argument when passed.
Only apply wait in Prefer header when return_async_id isn't provided.

Author: MariusWirtz

TM1py/Services/RestService.py

@@ -371,7 +371,11 @@ def _execute_async_request(
         """
         # Add async header
         http_headers = kwargs.get("headers", dict())
-        http_headers.update({"Prefer": "respond-async,wait=55"})
+        # if return_async_id is explicitly requested, we don't add wait time to Prefer header
+        if not return_async_id:
+            http_headers.update({"Prefer": "respond-async,wait=55"})
+        else:
+            http_headers.update({"Prefer": "respond-async"})
         kwargs["headers"] = http_headers
 
         # Make initial request