More minor fixes to melt, #1749.
More minor fixes to melt, #1749.

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