Add custom comparison for Montrose::Schedule

In our application using Montrose, we have some instances where we want
to be able to check to see if a schedule has changed.

The specific use case for us is with a Rails model that is serializing a
schedule - we want to be able to know if the schedule itself has
changed, ie: `event.schedule_changed? #=> false`

The same attributes may be assigned for the schedule's configuration -
but Rails thinks that the schedule has changed b/c it's comparing
the objects themselves instead of the underlying recurrence
configurations.

This commit overloads `Montrose::Schedule#==` so that schedules can be
compared against each other within the context of their configurations.

lib/montrose/schedule.rb

@@ -169,6 +169,14 @@ def inspect
       "#<#{self.class}:#{object_id.to_s(16)} #{to_a.inspect}>"
     end
 
+    def ==(other)
+      if !other.is_a?(self.class) && !other.is_a?(Array)
+        super
+      else
+        to_a == other.to_a
+      end
+    end
+
     private
 
     def active_enums(enums)

spec/montrose/schedule_spec.rb

@@ -337,4 +337,27 @@
       _(array).must_equal [{"every" => "month"}, {"every" => "day"}]
     end
   end
+
+  describe "#==" do
+    before do
+      schedule << {every: :month}
+      schedule << {every: :day}
+    end
+
+    it "compares two schedules against each other" do
+      identical_other = new_schedule.tap do |schedule|
+        schedule << {every: :month}
+        schedule << {every: :day}
+      end
+
+      different_other = new_schedule.tap do |schedule|
+        schedule << {every: :month}
+        schedule << {every: :day}
+        schedule << {every: :year}
+      end
+
+      _(schedule).must_equal identical_other
+      _(schedule).wont_equal different_other
+    end
+  end
 end