fake_omaha.py: Fix invalid full_update keyword

The correct key that clients and tests use is full_payload, not
full_update.

BUG=chromium:1157326
TEST=None

Change-Id: Ib0b1989f6a71d8d4c4d2c961f35ebbf248b24809
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/2593732
Commit-Queue: Amin Hassani <ahassani@chromium.org>
Commit-Queue: Kyle Shimabukuro <kyleshima@chromium.org>
Tested-by: Amin Hassani <ahassani@chromium.org>
Auto-Submit: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Sanika Kulkarni <sanikak@chromium.org>
Reviewed-by: Kyle Shimabukuro <kyleshima@chromium.org>
diff --git a/gs_cache/fake_omaha.py b/gs_cache/fake_omaha.py
index ea546c7..57ca934 100644
--- a/gs_cache/fake_omaha.py
+++ b/gs_cache/fake_omaha.py
@@ -30,7 +30,7 @@
   def POST(self, *args, **kwargs):
     """A URL handler to handle update check ping."""
     label = '/'.join(args)
-    full_update = kwargs.pop('full_update', 'unspecified')
+    full_update = kwargs.pop('full_payload', 'unspecified')
     server_addr, _ = cherrypy.request.headers.get('X-Forwarded-Host').split(':')
     body_length = int(cherrypy.request.headers.get('Content-Length', 0))
     data = cherrypy.request.rfile.read(body_length)