repr of PYSON Eval with a default PYSON expression ends in a traceback
```
Traceback (most recent call last):
File "/home/nicoe/projets/tryton/tryton/trytond/trytond/tests/test_pyson.py", line 69, in test_Eval_default
self.assertEqual(repr(expr), "(Eval('foo', Eval('bar', 1)) == 1)")
^^^^^^^^^^
File "/home/nicoe/projets/tryton/tryton/trytond/trytond/pyson.py", line 93, in __repr__
repr(params[0]), self._binary_operator, repr(params[1]))
^^^^^^^^^^^^^^^
File "/home/nicoe/projets/tryton/tryton/trytond/trytond/pyson.py", line 86, in __repr__
params = self.__repr_params__
^^^^^^^^^^^^^^^^^^^^
File "/home/nicoe/projets/tryton/tryton/trytond/trytond/pyson.py", line 152, in __repr_params__
if self._default != '':
^^^^^^^^^^^^^^^^^^^
File "/home/nicoe/projets/tryton/tryton/trytond/trytond/pyson.py", line 62, in __ne__
return Not(Equal(self, other))
^^^^^^^^^^^^^^^^^^
File "/home/nicoe/projets/tryton/tryton/trytond/trytond/pyson.py", line 314, in __init__
assert types1 == types2, 'statements must have the same type'
^^^^^^^^^^^^^^^^
AssertionError: statements must have the same type
```
issue