Fixing a bug for plotting an unbndfun

Fixing a bug for plotting an unbndfun defined on [a Inf] or [-Inf b] which has unbounded function value at the finite endpoint.
```
g = chebfun(@(x) 1./x.^2, [0 Inf], 'exps', [-2 0]); plot(g)
```
Before:
![fig1](https://cloud.githubusercontent.com/assets/3542751/4877143/6c2ef0ca-62de-11e4-8988-a902e908f046.png)
After:
![fig2](https://cloud.githubusercontent.com/assets/3542751/4877144/738b1114-62de-11e4-963b-9de6b7bc0b42.png)