syncml: use cutXML with str, not bytes

When cutting with bytes, we might cut in the middle of a multi-bytes
character and later decoding it would cause an error. We could switch
to sending bytes encoded to base64, because the limit would be more
natural to be number of bytes, but just decoding and cutting the string
is easier and in practice not so different.

This bug was revealed by testERP5SyncML.TestERP5SyncML.test_28_PartialData
with PYTHONHASHSEED 872 on python2

bt5/erp5_syncml/ModuleComponentTemplateItem/portal_components/module.erp5.XMLSyncUtils.py

@@ -192,13 +192,15 @@ def getXupdateObject(object_xml=None, old_xml=None):
 
 def cutXML(xml_string, length=None):
   """
-  Sliced a xml tree a return two fragment
+  Sliced a xml tree and return two fragments
   """
   if length is None:
     length = MAX_LEN
+  if not isinstance(xml_string, six.text_type):
+    xml_string = xml_string.decode('utf-8')
   short_string = xml_string[:length]
   rest_string = xml_string[length:]
-  xml_string = etree.CDATA(short_string.decode('utf-8'))
+  xml_string = etree.CDATA(short_string)
   return xml_string, rest_string
 
 class XMLSyncUtilsMixin(object):