Bug for FeatureEffects with method = "pdp" and categorical features

Minimal Example (see comments for the bug)
```r
library("mlr3")
library("mlr3learners")
library("iml")

credit.task = tsk("german_credit")
lrn = lrn("classif.ranger", predict_type = "prob")
model = lrn$train(credit.task)
m = Predictor$new(model, data = credit.task$data(), y = "credit_risk")

# BUG for method = "pdp" for categorical features:
pdp = FeatureEffects$new(m, feature = c("status", "duration"), method = "pdp")
plot(pdp)

# this works:
pdp = FeatureEffect$new(m, feature = "status", method = "pdp")
plot(pdp)

# this also works:
pdp = FeatureEffects$new(m, feature = c("status", "duration"), method = "pdp+ice")
plot(pdp)
```