Return counter value after incrementing/decrementing

- Redis multi return an array with the responses of each command that
 was run. Use [-1] to get the last value of the array and return it.

Author: pbstriker38

lib/kredis/types/counter.rb

@@ -7,14 +7,14 @@ def increment(by: 1)
     multi do
       set 0, ex: expires_in, nx: true
       incrby by
-    end
+    end[-1]
   end
 
   def decrement(by: 1)
     multi do
       set 0, ex: expires_in, nx: true
       decrby by
-    end
+    end[-1]
   end
 
   def value

test/types/counter_test.rb

@@ -12,27 +12,35 @@ class CounterTest < ActiveSupport::TestCase
 
     @counter.increment
     assert_equal 2, @counter.value
+
+    assert_equal 3, @counter.increment
   end
 
   test "increment by 2" do
     assert_equal 0, @counter.value
 
     @counter.increment by: 2
     assert_equal 2, @counter.value
+
+    assert_equal 4, @counter.increment(by: 2)
   end
 
   test "decrement" do
     assert_equal 0, @counter.value
 
     @counter.decrement
     assert_equal (-1), @counter.value
+
+    assert_equal (-2), @counter.decrement
   end
 
   test "decrement by 2" do
     assert_equal 0, @counter.value
 
     @counter.decrement by: 2
     assert_equal (-2), @counter.value
+
+    assert_equal (-4), @counter.decrement(by: 2)
   end
 
   test "expiring counter" do