Hotfix/lkj bug

This fixes the problem @bob-carpenter noticed here
https://groups.google.com/d/msg/stan-dev/5iEQ9mfmXVI/SdgxV-3GkYMJ
Now, Omega and Sigma have the same (and constant) marginals in this .stan file
```
transformed data {
  int K;
  real eta;
  K <- 4;
  eta <- 1;
}
parameters {
  corr_matrix[K] Omega;
}
model {

}
generated quantities {
  corr_matrix[K] Sigma;
  Sigma <- lkj_corr_rng(K,eta);
}
```