Revert "tweak: add Content-Type header to test request headers"

This reverts commit 37d6117.

Author: ahnv

tests/helpers.py

@@ -51,7 +51,7 @@ def setUp(self, mock_file, mock_req):
 
 
 def create_headers_for_test():
-    headers = {"Accept-Encoding": "gzip, deflate", "Content-Type": "application/json"}
+    headers = {"Accept-Encoding": "gzip, deflate"}
     headers.update(get_auth_headers_for_test())
     return headers