GaussianJacobi produces wrong eigenvalues

As reported by @dlfivefifty,

```
eigvalrand(GaussianJacobi(2,0,0),100)
```

produces eigenvalues shown in the plot. They should be symmetrically distributed around 1/2, so the heavy clustering should be present both at 0 and 1. Instead, the clustering only happens near 1.


Start by reproducing the reported eigvalrand(GaussianJacobi(2,0,0),100) result and inspect the GaussianJacobi and eigvalrand entry points. Determine why the eigenvalues cluster near 1 instead of symmetrically near 0 and 1; done means the distribution has the expected symmetry and the behavior is covered by a regression check.

Written by the indexing model from the issue text.