R 4.0 compatible head-tail tests, closes #4127 (#4136)
R 4.0 compatible head-tail tests, closes #4127 (#4136)

* R 4.0 compatible head-tail tests, closes #4127

* tail has extra dimnames arg

jangorecki/data.table,vlulla/data.table,jangorecki/data.table,jangorecki/data.table,vlulla/data.table,Rdatatable/data.table,vlulla/data.table,Rdatatable/data.table,jangorecki/data.table,Rdatatable/data.table,vlulla/data.table,Rdatatable/data.table