pytest_plugin: fix mutable kwarg defaults

Author: oliver-sanders

jupyter_server/pytest_plugin.py

@@ -316,7 +316,11 @@ async def my_test(jp_fetch):
             ...
     """
 
-    def client_fetch(*parts, headers={}, params={}, **kwargs):
+    def client_fetch(*parts, headers=None, params=None, **kwargs):
+        if not headers:
+            headers = {}
+        if not params:
+            params = {}
         # Handle URL strings
         path_url = url_escape(url_path_join(*parts), plus=False)
         base_path_url = url_path_join(jp_base_url, path_url)
@@ -358,7 +362,11 @@ async def my_test(jp_fetch, jp_ws_fetch):
             ...
     """
 
-    def client_fetch(*parts, headers={}, params={}, **kwargs):
+    def client_fetch(*parts, headers=None, params=None, **kwargs):
+        if not headers:
+            headers = {}
+        if not params:
+            params = {}
         # Handle URL strings
         path_url = url_escape(url_path_join(*parts), plus=False)
         base_path_url = url_path_join(jp_base_url, path_url)