Add last to lists

For simplicity, this does not strictly respect Array#last when nil or
false is passed, returning the last element instead of raising a
TypeError.

It also doesn't coerce the parameter into an int with to_int like
Array#last does.

lib/kredis/types/list.rb

@@ -24,4 +24,18 @@ def append(*elements)
   def clear
     del
   end
+
+  def last(n = nil)
+    if n
+      if n == 0
+        []
+      elsif n < 0
+        raise ArgumentError, "negative array size"
+      else
+        lrange(-n, -1)
+      end
+    else
+      lrange(-1, -1).first
+    end
+  end
 end

test/types/list_test.rb

@@ -41,6 +41,18 @@ class ListTest < ActiveSupport::TestCase
     assert_equal [], @list.elements
   end
 
+  test "last" do
+    @list.append(%w[ 1 2 3 ])
+    assert_equal "3", @list.last
+  end
+
+  test "last(n)" do
+    @list.append(%w[ 1 2 3 ])
+    assert_equal %w[ 2 3 ], @list.last(2)
+    assert_equal [], @list.last(0)
+    assert_raises(ArgumentError) { @list.last(-2) }
+  end
+
   test "typed as datetime" do
     @list = Kredis.list "mylist", typed: :datetime