Fix bug in resampling when using predict = "train" (#1284)

[MariaErdmann]

suggestion for NEWS:

## Bug fixes
* fixed bug issued in #1284, a bug in resample when using predict = "train"

[larskotthoff]

Thanks. Do the tests cover the case that you reported in the issue?

[MariaErdmann]

Do the tests cover the case that you reported in the issue?

I checked again and recognized I missed checking for predict.type and threshold.

I thought I check whether the entries in <resample.object>$pred for threshold and predict.type are not NULL which caused this error

Error in as.data.frame.default(x) : 
  cannot coerce class "c("ResamplePrediction", "NULL")" to a data.frame

and then I check whether there are entries in time which are not NA to catch this warning

In mean.default(x$time) : argument is not numeric or logical: returning NA

Moreover, I check whether the task description in the <resample.object>$pred is the same as the passed one.

[larskotthoff]

Thanks, merging.

[giuseppec]

@MariaErdmann don't forget to open up a new PR for the things we discussed and shortly summarize them.

[MariaErdmann]

@giuseppec : Yes, thank you, see #1324 .