Validate post ratings.

Prevent ratings from being set to invalid values via the API:

    PUT /posts/1.json?post[rating]=Z

app/models/post.rb

@@ -45,6 +45,7 @@ class Post < ActiveRecord::Base
   has_many :disapprovals, :class_name => "PostDisapproval", :dependent => :destroy
   has_many :favorites, :dependent => :destroy
   validates_uniqueness_of :md5
+  validates_inclusion_of :rating, in: %w(s q e), message: "rating must be s, q, or e"
   validate :post_is_not_its_own_parent
   attr_accessible :source, :rating, :tag_string, :old_tag_string, :old_parent_id, :old_source, :old_rating, :parent_id, :has_embedded_notes, :as => [:member, :builder, :gold, :platinum, :janitor, :moderator, :admin, :default]
   attr_accessible :is_rating_locked, :is_note_locked, :as => [:builder, :janitor, :moderator, :admin]