loo with k_threshold error for stan_polr

[jgabry]

loo with k_threshold errors for stan_polr because at this line

rstanarm/R/loo.R

Line 644 in bb59f7a

newz = x$z[omitted, , drop = FALSE], # NULL other than for some stan_betareg models

because of partial matching by $ (x$z partially matches the x$zeta in the object returned by stan_polr). This was initially reported on discourse (https://discourse.mc-stan.org/t/issue-with-loo-with-k-threshold/17052), albeit with a slightly different error message than I was getting.

I'm about to push a fix for this. @bgoodri Did you already submit the rstanarm patch with the stan_aov() fix? If not we should include this, otherwise it can wait until next release.