Return Time objects instead of deprecated DateTime (#106)

Time is preferred for current dates and times. DateTime is considered deprecated since Ruby 3.0: https://ruby-doc.org/stdlib-3.0.0/libdoc/date/rdoc/DateTime.html

Author: bdewater

lib/kredis/type/datetime.rb

@@ -8,7 +8,7 @@ def serialize(value)
       end
 
       def cast_value(value)
-        super&.to_datetime
+        super&.to_time
       end
     end
   end

test/types/scalar_test.rb

@@ -53,7 +53,7 @@ class ScalarTest < ActiveSupport::TestCase
   test "datetime casting Dates" do
     datetime = Kredis.datetime "myscalar"
     datetime.value = Date.current
-    assert_equal Date.current.to_datetime, datetime.value
+    assert_equal Date.current.to_time, datetime.value
   end
 
   test "json" do