ggpredict doesn't work for vglm models where multiple.responses = TRUE

[B0ydT]

I've been messing around with multivariate models in the 'VGAM' package, and have had some trouble with ggeffects as illustrated:

library(VGAM)
library(ggeffects)

data("hunua")
shunua <- hunua[sort.list(with(hunua, altitude)), ]

fit <- vglm(cbind(agaaus, kniexc) ~ altitude,
             binomialff(multiple.responses = TRUE), data = shunua)

ggpredict(fit, "altitude [all]")

# Error in `$<-.data.frame`(`*tmp*`, "predicted", value = c(0.217797358721427,  : 
#   replacement has 128 rows, data has 64

I've got it working locally with some minor changes to get_predictions_vglm (checking if multiple responses are expected) and will submit a PR shortly. I aimed to only change the behaviour where multiple.responses = TRUE, but I'm still quite new to VGAM so I'm more than happy to make any suggested changes as necessary.

Thanks for your time if you get a chance to look at this, and thanks for an awesome package regardless!

[strengejacke]

I added a test, let's see if this passes.