inconsistencies between merge.data.frame and merge.data.table

[MarkusBonsch]

We currently have several issues open that are concerned with the differences in behavior between merge.data.table and merge.data.frame. This issue is aimed at collecting all of them in order to facilitate fast and efficient solutions without duplicated work.

• joins with type coercion: 1.5 == 1 is TRUE #2592 (severe bug imho)
• merge.data.table ignores incomparables without warnings. #2587 merge.data.table ignores incomparables without warnings
• sort = FALSE does't generate equivalent results with data.frame.merge #2574 sort = FALSE does't generate equivalent results with data.frame.merge
• forderv orders NA's different than base::order by default #2594 forderv orders NAs differently than base::order by default