Fix Rdatatable#1347, dispatch if last masks xts::last

data.table::last is intended to work with xts::last, which is a S3
generic. e011351 appears to have caused a regression that prevents
xts::last from dispatching.

Revert e011351, but keep the is.data.frame check on the happy path, so
it will be fast for data.table objects. Return xts::last if xts is on
the search path. Otherwise, return tail(x, ...) instead of throwing an
error (the previous behavior).

R/last.R

@@ -4,10 +4,14 @@
 # We'd like last() on vectors to be fast, so that's a direct x[NROW(x)] as it was in data.table, otherwise use xts's.
 # If xts is loaded higher than data.table, xts::last will work but slower.
 last <- function(x, ...) {
-    if (nargs()==1L || !"package:xts" %in% search()) {
+    if (nargs()==1L) {
         if (is.data.frame(x)) return(x[NROW(x),])
-        if (!length(x)) return(x) else return(x[[length(x)]])  # for vectors, [[ works like [
+        if (is.vector(x)) {
+            if (!length(x)) return(x) else return(x[[length(x)]])  # for vectors, [[ works like [
+        }
     }
-    xts::last(x,...)   # UseMethod("last") doesn't find xts's methods, not sure what I did wrong.
+    if ("package:xts" %in% search())
+        return(xts::last(x,...))
+    tail(x, ...)
 }