fix: remove content-type header from idms get requests

Author: peter-volkov

google/auth/aws.py

@@ -650,7 +650,7 @@ def _get_metadata_security_credentials(
             google.auth.exceptions.RefreshError: If an error occurs while
                 retrieving the AWS security credentials.
         """
-        headers = {"Content-Type": "application/json"}
+        headers = {}
         if imdsv2_session_token is not None:
             headers["X-aws-ec2-metadata-token"] = imdsv2_session_token