define hash on duration

activesupport/lib/active_support/duration.rb

@@ -63,6 +63,10 @@ module ActiveSupport
       Duration === other && other.value.eql?(value)
     end
 
+    def hash
+      @value.hash
+    end
+
     def self.===(other) #:nodoc:
       other.is_a?(Duration)
     rescue ::NoMethodError

activesupport/test/core_ext/duration_test.rb

@@ -196,4 +196,8 @@ class DurationTest < ActiveSupport::TestCase
     assert_respond_to 1.day, :since
     assert_respond_to 1.day, :zero?
   end
+
+  def test_hash
+    assert_equal 1.minute.hash, 60.seconds.hash
+  end
 end