tests: add check for backward compatibility of validation attribute

tests/test_formdef_import.py

@@ -535,6 +535,15 @@ def test_field_validation():
     assert len(f2.fields) == len(formdef.fields)
     assert f2.fields[0].validation == formdef.fields[0].validation
 
+    # backward compatibility
+    formdef_xml = formdef.export_to_xml()
+    old_format = ET.tostring(formdef_xml).replace(
+            '<validation><type>regex</type><value>\\d</value></validation>',
+            '<validation>\\d</validation>')
+    f2 = FormDef.import_from_xml(StringIO.StringIO(old_format))
+    assert len(f2.fields) == len(formdef.fields)
+    assert f2.fields[0].validation == {'type': 'regex', 'value': '\\d'}
+
 def test_digest_template():
     formdef = FormDef()
     formdef.name = 'Foo'