Don't convert issuable_initial_data into JSON

Instead of converting hash into JSON inside
issuable_initial_data method, return hash and convert
to JSON later. This allows us to easily extend basic
issuable data with resource specific values. For example
for Epic these data should include also labels, so we can then
do something like:
issuable_initial_data(@epic).merge(labels: @epic.labels).to_json

app/helpers/issuables_helper.rb

@@ -234,7 +234,7 @@ module IssuablesHelper
 
     data.merge!(updated_at_by(issuable))
 
-    data.to_json
+    data
   end
 
   def updated_at_by(issuable)

app/views/projects/issues/show.html.haml

@@ -55,7 +55,7 @@
 
 .issue-details.issuable-details
   .detail-page-description.content-block
-    %script#js-issuable-app-initial-data{ type: "application/json" }= issuable_initial_data(@issue)
+    %script#js-issuable-app-initial-data{ type: "application/json" }= issuable_initial_data(@issue).to_json
     #js-issuable-app
       %h2.title= markdown_field(@issue, :title)
       - if @issue.description.present?

spec/helpers/issuables_helper_spec.rb

@@ -173,23 +173,23 @@ describe IssuablesHelper do
       @project = issue.project
 
       expected_data = {
-        'endpoint' => "/#{@project.full_path}/issues/#{issue.iid}",
-        'updateEndpoint' => "/#{@project.full_path}/issues/#{issue.iid}.json",
-        'canUpdate' => true,
-        'canDestroy' => true,
-        'issuableRef' => "##{issue.iid}",
-        'markdownPreviewPath' => "/#{@project.full_path}/preview_markdown",
-        'markdownDocsPath' => '/help/user/markdown',
-        'issuableTemplates' => [],
-        'projectPath' => @project.path,
-        'projectNamespace' => @project.namespace.path,
-        'initialTitleHtml' => issue.title,
-        'initialTitleText' => issue.title,
-        'initialDescriptionHtml' => '<p dir="auto">issue text</p>',
-        'initialDescriptionText' => 'issue text',
-        'initialTaskStatus' => '0 of 0 tasks completed'
+        endpoint: "/#{@project.full_path}/issues/#{issue.iid}",
+        updateEndpoint: "/#{@project.full_path}/issues/#{issue.iid}.json",
+        canUpdate: true,
+        canDestroy: true,
+        issuableRef: "##{issue.iid}",
+        markdownPreviewPath: "/#{@project.full_path}/preview_markdown",
+        markdownDocsPath: '/help/user/markdown',
+        issuableTemplates: [],
+        projectPath: @project.path,
+        projectNamespace: @project.namespace.path,
+        initialTitleHtml: issue.title,
+        initialTitleText: issue.title,
+        initialDescriptionHtml: '<p dir="auto">issue text</p>',
+        initialDescriptionText: 'issue text',
+        initialTaskStatus: '0 of 0 tasks completed'
       }
-      expect(JSON.parse(helper.issuable_initial_data(issue))).to eq(expected_data)
+      expect(helper.issuable_initial_data(issue)).to eq(expected_data)
     end
   end