:= should drop secondary key

[mattdowle]

As raised here:
http://stackoverflow.com/questions/26308072/operator-inconsistent-in-logical-columns-in-data-table/26308820

[arunsrinivasan]

I'm guessing that includes set() as well.

Also the same happens with setorder(). setkey() seems to handle it properly though.

Here's an example:

set.seed(45L)
require(data.table)
dt = data.table(x=sample(3,20, TRUE), y=sample(4,20,TRUE))
dt[x==1L] ## 9 rows
dt[x==2L]
attributes(dt)
setorder(dt, -y)
attributes(dt)
dt[x==1L] ## 1 row