remove OpenStruct usage from AccessToken

Change-Id: I3a203d298a01e2e6f355a0cd4e401ecd42fb37c9
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/345275
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: Weston Dransfield <wdransfield@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>

app/models/access_token.rb

@@ -65,12 +65,7 @@ class AccessToken < ActiveRecord::Base
   scope :not_deleted, -> { where(workflow_state: "active") }
 
   TOKEN_SIZE = 64
-  TOKEN_TYPES = OpenStruct.new(
-    {
-      crypted_token: :crypted_token,
-      crypted_refresh_token: :crypted_refresh_token
-    }
-  )
+  TOKEN_TYPES = [:crypted_token, :crypted_refresh_token].freeze
 
   before_create :generate_token
   before_create :generate_refresh_token

spec/models/access_token_spec.rb

@@ -68,7 +68,7 @@
     end
 
     context "when an access token argument is provided" do
-      subject { AccessToken.authenticate(token.full_token, AccessToken::TOKEN_TYPES.crypted_token, token) }
+      subject { AccessToken.authenticate(token.full_token, :crypted_token, token) }
 
       let(:token) { AccessToken.create!(user: user_model) }
       let(:user) { user_model }