Treat combined durations as a single unit

Prior to this commit, `3.months - 3.months` would result in a duration
that has the "parts" of `[[:months, 3], [:months, -3]]`. This would mean
that it was subtly different than `2.months - 2.months`. When applied to
a time, the date might actually change if the resulting day doesn't
exist however many months in the future, even though in both cases we
just wanted to add `0`, which should always be an identity operation.

With this change, we now store the parts as a hash, so `3.months -
3.months` is simply stored as `{ months: 0 }`.

activesupport/lib/active_support/duration.rb

@@ -15,16 +15,22 @@ class Duration
     autoload :ISO8601Serializer, "active_support/duration/iso8601_serializer"
 
     def initialize(value, parts) #:nodoc:
-      @value, @parts = value, parts
+      @value, @parts = value, parts.to_h
+      @parts.default = 0
     end
 
     # Adds another Duration or a Numeric to this Duration. Numeric values
     # are treated as seconds.
     def +(other)
       if Duration === other
-        Duration.new(value + other.value, @parts + other.parts)
+        parts = @parts.dup
+        other.parts.each do |(key, value)|
+          parts[key] += value
+        end
+        Duration.new(value + other.value, parts)
       else
-        Duration.new(value + other, @parts + [[:seconds, other]])
+        seconds = @parts[:seconds] + other
+        Duration.new(value + other, @parts.merge(seconds: seconds))
       end
     end
 

activesupport/test/core_ext/duration_test.rb

@@ -345,6 +345,17 @@ def test_iso8601_parsing_across_autumn_dst_boundary
     end
   end
 
+  def test_adding_durations_do_not_hold_prior_states
+    time = Time.parse("Nov 29, 2016")
+    # If the implementation adds and subtracts 3 months, the
+    # resulting date would have been in February so the day will
+    # change to the 29th.
+    d1 = 3.months - 3.months
+    d2 = 2.months - 2.months
+
+    assert_equal time + d1, time + d2
+  end
+
   private
     def eastern_time_zone
       if Gem.win_platform?