Closes #893. dcast handles factor levels correctly on drop=FALSE.
Minor: use is.data.table()
Plug in setDT, also takes care of null data.table.
Removing 'droplevels' in melt. Dint come up as a FR.
Skip 0-length inputs in dt_na, to use in melt as well.
Remove 'static' from combineFactorLevels, to use in 'melt'.
melt can, well, melt into multiple columns now!
Fixed a segfault in handling na.rm argument in melt.
Modified existing tests and added more for melt.
Set 'sorted' attribute to NULL in 'melt'.
Updated documention and added examples.
Merge branch 'melt-multiple' into 1.9.8
which_ - mem efficient version of which(!x)