fix: removes content-header-from-aws-get

Author: chalmerlowe

google/auth/aws.py

@@ -530,7 +530,7 @@ 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 = None
         if imdsv2_session_token is not None:
             headers["X-aws-ec2-metadata-token"] = imdsv2_session_token