Fix github driver tests for py3

Urllib needs bytes for POST data.

Change-Id: I5cab622c50dbc50892526416944f52a8cd4364a7
diff --git a/tests/base.py b/tests/base.py
index b668322..21a5892 100755
--- a/tests/base.py
+++ b/tests/base.py
@@ -840,7 +840,7 @@
         """Emulates sending the GitHub webhook event to the connection."""
         port = self.webapp.server.socket.getsockname()[1]
         name, data = event
-        payload = json.dumps(data)
+        payload = json.dumps(data).encode('utf8')
         headers = {'X-Github-Event': name}
         req = urllib.request.Request(
             'http://localhost:%s/connection/%s/payload'