add destructive set_broadcast_policy! to BroadcastPolicy.

fixes #CNVS-2732

current implementation of set_broadcast_policy shares
notifications across parent and child classes; this method
provides a way for child classes to choose not to share
their notifications with the parent.

test plan:
  * create a user and set her notification policies for
    announcements and discussions to ASAP;
  * create a new discussion;
  * verify that the user receives a single message about
    the new discussion;
  * repeat the same process with an announcement.

Change-Id: Iaafa97f550f804d8bac4fb95552a6a4eeb2626dd
Reviewed-on: https://gerrit.instructure.com/16935
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Mark Ericksen <marke@instructure.com>
QA-Review: Marc LeGendre <marc@instructure.com>

Author: zachpendleton

app/models/announcement.rb

@@ -34,16 +34,16 @@ def infer_content
     self.title ||= t(:no_title, "No Title")
   end
   protected :infer_content
-  
-  set_broadcast_policy do 
+
+  set_broadcast_policy! do
     dispatch :new_announcement
     to { active_participants(true) - [user] }
-    whenever { |record| 
+    whenever { |record|
       record.context.available? and
       ((record.just_created and not record.post_delayed?) || record.changed_state(:active, :post_delayed))
     }
   end
-    
+
   set_policy do
     given { |user| self.user == user }
     can :update and can :reply and can :read

vendor/plugins/broadcast_policy/lib/broadcast_policy.rb

@@ -237,6 +237,11 @@ def set_broadcast_policy(&block)
         self.broadcast_policy_list.populate(&block)
       end
 
+      def set_broadcast_policy!(&block)
+        self.broadcast_policy_list = PolicyList.new
+        self.broadcast_policy_list.populate(&block)
+      end
+
     end # SingletonMethods
 
     module InstanceMethods

vendor/plugins/broadcast_policy/spec_canvas/broadcast_policy_spec.rb

@@ -64,6 +64,25 @@ class AnotherModel
     list.find_policy_for(foo).should be_present
     list.find_policy_for(bar).should be_present
   end
+
+  it "should allow overwriting of the current block w/ set_broadcast_policy!" do
+    foo = Canvas::MessageHelper.create_notification('Foo', 'Foo', 0, '', 'Foo')
+    bar = Canvas::MessageHelper.create_notification('Bar', 'Bar', 0, '', 'Bar')
+
+    class Parent < AnotherModel
+      has_a_broadcast_policy
+
+      set_broadcast_policy { dispatch :foo; to {}; whenever {} }
+    end
+
+    class Child < Parent
+      has_a_broadcast_policy
+
+      set_broadcast_policy! { }
+    end
+
+    Child.broadcast_policy_list.find_policy_for(foo).should_not be_present
+  end
   # it "should require a block" do
   #   lambda{
   #     class AnotherModel