--- handlers.py	(revision 66389)
+++ handlers.py	(working copy)
@@ -248,7 +248,7 @@
     def finish_content(self):
         """Ensure headers and content have both been sent"""
         if not self.headers_sent:
-            self.headers['Content-Length'] = "0"
+            self.headers.setdefault('Content-Length', "0")
             self.send_headers()
         else:
             pass # XXX check if content-length was too short?