Fix incorrect timezone of -0000. Use GMT instead. Error found using redbot

Author: rouilj

CHANGES.txt

@@ -159,6 +159,8 @@ Fixed:
   Rouillard)
 - report basename of filename when template file is invalid
   rather than reporting a TypeError. (John Rouillard)
+- Make Last-Modified header use GMT not -0000 timezone. Fix error
+  reported by redbot testing. (John Rouillard)
 
 Features:
 

roundup/cgi/client.py

@@ -1984,7 +1984,8 @@ def _serve_file(self, lmt, mime_type, content=None, filename=None):
         """
 
         # spit out headers
-        self.additional_headers['Last-Modified'] = email.utils.formatdate(lmt)
+        self.additional_headers['Last-Modified'] = email.utils.formatdate(lmt,
+	 usegmt=True)
 
         ims = None
         # see if there's an if-modified-since...