Fixed #6727 -- Made patch_cache_control() patch an empty Cache-Control header.

django/utils/cache.py

@@ -63,7 +63,7 @@ def patch_cache_control(response, **kwargs):
         else:
             return '%s=%s' % (t[0], t[1])
 
-    if response.has_header('Cache-Control'):
+    if response.get('Cache-Control'):
         cc = cc_delim_re.split(response['Cache-Control'])
         cc = dict(dictitem(el) for el in cc)
     else:

tests/cache/tests.py

@@ -1456,6 +1456,7 @@ class CacheUtils(SimpleTestCase):
         tests = (
             # Initial Cache-Control, kwargs to patch_cache_control, expected Cache-Control parts
             (None, {'private': True}, {'private'}),
+            ('', {'private': True}, {'private'}),
 
             # Test whether private/public attributes are mutually exclusive
             ('private', {'private': True}, {'private'}),