Bug in Expression.user_parameters interface

[Jan Blechta]

from dolfin import *
e = Expression('u', u=1, degree=1)

e.user_parameters['u'] = 2
assert e.u == 2.0
assert e(0.0) == 2.0

e.u = 4
assert e(0.0) == 4.0
assert e.user_parameters['u'] == 4.0  # Fails!