add test for TRUELENGTH=0 set

Author: ben-schwen

inst/tests/tests.Rraw

@@ -18709,4 +18709,6 @@ n = getOption("datatable.alloccol")
 DT = data.table()
 for (i in seq(10*n)) set(DT, j = paste0("V",i), value = i)
 test(2235.1, ncol(DT), 10L*n)
+DT = structure(list(a = 1, b = 2), class = c("data.table", "data.frame"))
+test(2235.2, set(DT, j="c", value=3), data.table(a=1, b=2, c=3))
 options(opt)