Use polymorphism to implement allows_multiple_assignees?

Only Issue might allow multiple assignees, but only when the
multiple_issue_assignees feature is available. Do this check in Issue
itself, instead of a type-based check in Issuable.

app/models/concerns/ee/issuable.rb

-module EE
-  module Issuable
-    extend ActiveSupport::Concern
-
-    def allows_multiple_assignees?
-      is_a?(Issue) && project.feature_available?(:multiple_issue_assignees)
-    end
-  end
-end

app/models/concerns/issuable.rb

@@ -23,8 +23,6 @@ module Issuable
   IssuableMeta = Struct.new(:upvotes, :downvotes, :notes_count, :merge_requests_count)
 
   included do
-    prepend EE::Issuable
-
     cache_markdown_field :title, pipeline: :single_line
     cache_markdown_field :description, issuable_state_filter_enabled: true
 

app/models/ee/issue.rb

@@ -5,6 +5,11 @@ module EE
       author.support_bot? || super
     end
 
+    # override
+    def allows_multiple_assignees?
+      project.feature_available?(:multiple_issue_assignees)
+    end
+
     # override
     def subscribed_without_subscriptions?(user, *)
       # TODO: this really shouldn't be necessary, because the support

spec/models/ee/issue_spec.rb

 require 'spec_helper'
 
 describe Issue do
+  describe '#allows_multiple_assignees?' do
+    it 'does not allow multiple assignees without license' do
+      stub_licensed_features(multiple_issue_assignees: false)
+
+      issue = build(:issue)
+
+      expect(issue.allows_multiple_assignees?).to be_falsey
+    end
+
+    it 'does not allow multiple assignees without license' do
+      stub_licensed_features(multiple_issue_assignees: true)
+
+      issue = build(:issue)
+
+      expect(issue.allows_multiple_assignees?).to be_truthy
+    end
+  end
+
   describe '#weight' do
     [
       { license: true,  database: 5,    expected: 5 },