Update httpcache.py

It checks cache directory modification time, but have to check file modification time.

scrapy/extensions/httpcache.py

@@ -328,7 +328,7 @@ class FilesystemCacheStorage(object):
         metapath = os.path.join(rpath, 'pickled_meta')
         if not os.path.exists(metapath):
             return  # not found
-        mtime = os.stat(rpath).st_mtime
+        mtime = os.stat(metapath).st_mtime
         if 0 < self.expiration_secs < time() - mtime:
             return  # expired
         with self._open(metapath, 'rb') as f: