let iso8601date handle strings

src/collective/solr/mangler.py

@@ -32,6 +32,8 @@ ignored = 'use_solr', '-C'
 
 def iso8601date(value):
     """ convert `DateTime` to iso 8601 date format """
+    if isinstance(value, str):
+        value = DateTime(value, datefmt='international')
     if isinstance(value, DateTime):
         v = value.toZone('UTC')
         value = '%04d-%02d-%02dT%02d:%02d:%06.3fZ' % (