Improve handling of str values for Duration

Passing a str to `Duration.xmlvalue(value)` will result
in a properly formatted iso duration.

Author: backbord

src/zeep/xsd/types/builtins.py

@@ -124,10 +124,12 @@ def pythonvalue(self, value):
 
 class Duration(BuiltinType):
     _default_qname = xsd_ns("duration")
-    accepted_types = [isodate.duration.Duration, str]
+    accepted_types = [isodate.duration.Duration, datetime.timedelta, str]
 
     @check_no_collection
     def xmlvalue(self, value):
+        if isinstance(value, str):
+            value = isodate.parse_duration(value)
         return isodate.duration_isoformat(value)
 
     @treat_whitespace("collapse")

tests/test_xsd_builtins.py

@@ -139,6 +139,10 @@ def test_xmlvalue(self):
         instance = builtins.Duration()
         value = isodate.parse_duration("P0Y1347M0D")
         assert instance.xmlvalue(value) == "P1347M"
+        assert instance.xmlvalue("P0Y1347M0D") == "P1347M"
+        assert instance.xmlvalue(datetime.timedelta(days=1347)) == "P1347D"
+        with pytest.raises(ValueError):
+            instance.xmlvalue("P15T")
 
     def test_pythonvalue(self):
         instance = builtins.Duration()