Treat Expression('1.0') and Expression(('1.0',)) the same

This makes us slight more lenient compared to DOLFIN on raising
exceptions on Expressions i.e. we will allow the following, which
would raise an exception in DOLFIN:

  VV = VectorFunctionSpace(UnitIntervalMesh(2), 'CG', 1)
  f = Function(VV).interpolate(Expression('1.0'))

Therefore the test that ensure this exception is raised is changed.
Add test for facet normals on the UnitIntervalMesh

It appears the normals are pointing in the wrong direction, though
the boundary markers on the UnitIntervalMesh appear to be correct.