DEV: Include raw in post webhook serializer

This came in as a request on meta to include the raw field in the post
webhook serializer.

 https://meta.discourse.org/t/-/49045/55?u=blake

 Including this field can prevent needing to make a 2nd API request to
 get the raw field of a post.

 It would be handy down the road if we updated the webhook ui to specify
 fields or arguments that you wanted to be included in the serialized
 data, but most requests I've seen to update the serializers have been
 valid requests that are good to add anyways, so I don't think we have
 reached that point yet.

app/serializers/web_hook_post_serializer.rb

@@ -11,6 +11,10 @@ class WebHookPostSerializer < PostSerializer
     true
   end
 
+  def include_raw?
+    true
+  end
+
   %i{
     can_view
     can_edit

spec/serializers/web_hook_post_serializer_spec.rb

@@ -12,7 +12,7 @@ RSpec.describe WebHookPostSerializer do
 
   it 'should only include the required keys' do
     count = serialized_for_user(admin).keys.count
-    difference = count - 39
+    difference = count - 40
 
     expect(difference).to eq(0), lambda {
       message = +""