addConstraint ignored

I was wondering why the constraint "x[0] == 1" is violated in the following program:

``` Python
m = CyClpSimplex()
x = m.addVariable('x', 2)
m.addConstraint( x[0] == 1 )
# does not work either:
# m += x[0] == 1
# this works:
# m += scipy.matrix([1, 0])*x == 1
m.objective = x[0]
m += 0 <= x[0]
cbcModel = m.getCbcModel()
cbcModel.solve()
print(cbcModel.primalVariableSolution['x'])
# output: [ 0. 0.]
```

It works when I remove the nonnegativity constraint or when I use the scipy.matrix constraint. Note that I use two variables instead of 1 for this example due to Issue #22.