Fixing Variables in foreign functions not unifiable

When using Variable in foreign_functions, the Unification did not change anything on the Variable.
This was also reproducable in examples/register_foreign.py where the output was:

python examples/register_foreign.py 
[{'Y': Variable(68), 'X': 'Python'}]

After patching it yield as expected:
[{'Y': 130, 'X': 'Python'}]