Fix partial match of $forest$split.values

[jemus42]

Avoids

#> Warning in arf$forest$split.value: partial match of 'split.value' to
#> 'split.values'

when enabling partial match warnings, e.g. in .Rprofile via

options(
  warnPartialMatchArgs = TRUE,
  warnPartialMatchDollar = TRUE,
  warnPartialMatchAttr = TRUE
)