Assembling Matrix on mixed x non-mixed space fails.

````python
m = UnitTriangleMesh()

V = FunctionSpace(m, 'DG', 0)
W = V*V

v = TestFunction(V)
w = TrialFunction(W)

a = v*w[0]*dx

A = assemble(a)

A.M._force_evaluation()

=>
ValueError: setting an array element with a sequence.
````