Don't invoke application default credentials logic when developerKey is provided (#347)

diff --git a/googleapiclient/discovery.py b/googleapiclient/discovery.py
index 6291f34..1266883 100644
--- a/googleapiclient/discovery.py
+++ b/googleapiclient/discovery.py
@@ -355,8 +355,9 @@
     scopes = list(
       service.get('auth', {}).get('oauth2', {}).get('scopes', {}).keys())
 
-    # If so, then the we need to setup authentication.
-    if scopes:
+    # If so, then the we need to setup authentication if no developerKey is
+    # specified.
+    if scopes and not developerKey:
       # If the user didn't pass in credentials, attempt to acquire application
       # default credentials.
       if credentials is None: