Don't optimise if .I, e.g. c(lapply(.SD, sum), .I)
Don't optimise c(…) in `j` if not wrapped by list().
In j-expr with c(..), .I if unnamed gets named "I".
Merge branch 'Fix-861'

Conflicts:
	inst/tests/tests.Rraw
Update NEWS
Merge branch 'Fix-861'
Merge branch 'master' into Feature-735
Fix test again - got lost during branch merge.
Merge branch 'master' into Feature-735
Fixed bug in mean -> fastmean during #735. Added test.
Merge branch 'Feature-735'