Fix ID token verification (#87)

diff --git a/google/oauth2/id_token.py b/google/oauth2/id_token.py
index 968f271..fa96fc0 100644
--- a/google/oauth2/id_token.py
+++ b/google/oauth2/id_token.py
@@ -47,7 +47,7 @@
         Mapping[str, str]: A mapping of public key ID to x.509 certificate
             data.
     """
-    response = request('GET', certs_url)
+    response = request(certs_url, method='GET')
 
     if response.status != http_client.OK:
         raise exceptions.TransportError(