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 and test_28_PartialData with PYTHONHASHSEED 238
( on python2 )