n.trees error with variable_response() function on gbm object

[kransom14]

This issue is related to a previous one: #4

I get an n.trees error even when passing the n.trees argument to variable_response() with an explainer created on a gbm model object.

library(gbm)
library(DALEX)
library(breakDown)

# create a gbm model
model <- gbm(quality ~ pH + residual.sugar + sulphates + alcohol, data = wine,
             distribution = "gaussian",
             n.trees = 1000,
             interaction.depth = 4,
             shrinkage = 0.01,
             n.minobsinnode = 10,
             verbose = FALSE)

# make an explainer for the model
explainer_gbm <- explain(model, data = wine)

# single variable
exp_sgn <- variable_response(explainer_gbm, variable = "alcohol", n.trees = 1000)

Error in paste("Using", n.trees, "trees...\n") : 
  argument "n.trees" is missing, with no default](url)

[kransom14]

I realized this issue has already been addressed for variable_importance() and has the same solution: #35