favicon fetcher: provide an Accept header on requests.

This makes the fetcher work with the default ModSecurity ruleset, and would also avoid providing unsupported favicon formats (e.g. webp), by explictly preferring PNG and
x-icon formats.

apps/rss_feeds/icon_importer.py

@@ -269,6 +269,7 @@ class IconImporter(object):
                                   self.feed.permalink
                               ),
                 'Connection': 'close',
+                'Accept': 'image/png,image/x-icon,image/*;q=0.9,*/*;q=0.8'
             }
             try:
                 request = urllib2.Request(url, headers=headers)