Avoid double evaluation from inherit with different model

safe_eval forbid to evaluate '__' but PYSON encoded contains such strings. So
with inherit from different model, the XML attributes could already been
encoded we should prevent to evaluate them a second times because it will fail.

issue4228
review5681002
(grafted from a12b87a6fe8b3e5e6ecefd58dfb565fda25341b2)

--HG--
branch : 2.6
extra : source : a12b87a6fe8b3e5e6ecefd58dfb565fda25341b2