Use datetime.utcnow to be consistent with other headers

Fixes: #158

Author: Eric Larson

cachecontrol/caches/redis_cache.py

@@ -25,7 +25,7 @@ def set(self, key, value, expires=None):
         if not expires:
             self.conn.set(key, value)
         else:
-            expires = expires - datetime.now()
+            expires = expires - datetime.utcnow()
             self.conn.setex(key, total_seconds(expires), value)
 
     def delete(self, key):

cachecontrol/heuristics.py

@@ -9,7 +9,7 @@
 
 
 def expire_after(delta, date=None):
-    date = date or datetime.now()
+    date = date or datetime.utcnow()
     return date + delta