More Enumerable `first` and `last` for `ActiveRecord::Result`

`Result#first` was added at #25507. As I said at https://github.com/rails/rails/pull/25507#issuecomment-228446339,
I still prefer less code, `Result#first` is used only in `select_one`,
and `select_one` is not used in the codebase, i.e. `Result#first` and
`Result#last` are not used in the codebase. I'd not prefer to have
dedicated code for such like rarely used (almost unused) feature.

Instead of having limited `Result#first` (doesn't allow number), I'd
prefer to just use `Enumerable#first`.

activerecord/lib/active_record/result.rb

@@ -92,18 +92,9 @@ def [](idx)
       hash_rows[idx]
     end
 
-    # Returns the first record from the rows collection.
-    # If the rows collection is empty, returns +nil+.
-    def first
-      return nil if @rows.empty?
-      Hash[@columns.zip(@rows.first)]
-    end
-
     # Returns the last record from the rows collection.
-    # If the rows collection is empty, returns +nil+.
-    def last
-      return nil if @rows.empty?
-      Hash[@columns.zip(@rows.last)]
+    def last(n = nil)
+      n ? hash_rows.last(n) : hash_rows.last
     end
 
     def cast_values(type_overrides = {}) # :nodoc:

activerecord/test/cases/result_test.rb

@@ -42,11 +42,35 @@ def result
     test "first returns first row as a hash" do
       assert_equal(
         { "col_1" => "row 1 col 1", "col_2" => "row 1 col 2" }, result.first)
+      assert_equal [
+        { "col_1" => "row 1 col 1", "col_2" => "row 1 col 2" },
+      ], result.first(1)
+      assert_equal [
+        { "col_1" => "row 1 col 1", "col_2" => "row 1 col 2" },
+        { "col_1" => "row 2 col 1", "col_2" => "row 2 col 2" },
+      ], result.first(2)
+      assert_equal [
+        { "col_1" => "row 1 col 1", "col_2" => "row 1 col 2" },
+        { "col_1" => "row 2 col 1", "col_2" => "row 2 col 2" },
+        { "col_1" => "row 3 col 1", "col_2" => "row 3 col 2" },
+      ], result.first(3)
     end
 
     test "last returns last row as a hash" do
       assert_equal(
         { "col_1" => "row 3 col 1", "col_2" => "row 3 col 2" }, result.last)
+      assert_equal [
+        { "col_1" => "row 3 col 1", "col_2" => "row 3 col 2" },
+      ], result.last(1)
+      assert_equal [
+        { "col_1" => "row 2 col 1", "col_2" => "row 2 col 2" },
+        { "col_1" => "row 3 col 1", "col_2" => "row 3 col 2" },
+      ], result.last(2)
+      assert_equal [
+        { "col_1" => "row 1 col 1", "col_2" => "row 1 col 2" },
+        { "col_1" => "row 2 col 1", "col_2" => "row 2 col 2" },
+        { "col_1" => "row 3 col 1", "col_2" => "row 3 col 2" },
+      ], result.last(3)
     end
 
     test "each with block returns row hashes" do