Make `is_supported_content` to return a boolean string

There is a specific behavior how HAML treats booleans.

* If value is true, then the data attribute key is present but
it has no value.
* If value is false, then the data attribute key is missing.

Such behavior makes it more difficult to fetch data attributes on
frontend side. It seems to be a common approach in GitLab codebase to
use boolean strings and parse them.

lib/gitlab/static_site_editor/config.rb

@@ -22,7 +22,7 @@ module Gitlab
           project: project.path,
           namespace: project.namespace.path,
           return_url: return_url,
-          is_supported_content: supported_content?
+          is_supported_content: supported_content?.to_s
         }
       end
 

spec/lib/gitlab/static_site_editor/config_spec.rb

@@ -24,38 +24,38 @@ describe Gitlab::StaticSiteEditor::Config do
         project: 'project',
         project_id: project.id,
         return_url: 'http://example.com',
-        is_supported_content: true
+        is_supported_content: 'true'
       )
     end
 
     context 'when branch is not master' do
       let(:ref) { 'my-branch' }
 
-      it { is_expected.to include(is_supported_content: false) }
+      it { is_expected.to include(is_supported_content: 'false') }
     end
 
     context 'when file does not have a markdown extension' do
       let(:file_path) { 'README.txt' }
 
-      it { is_expected.to include(is_supported_content: false) }
+      it { is_expected.to include(is_supported_content: 'false') }
     end
 
     context 'when file does not have an extension' do
       let(:file_path) { 'README' }
 
-      it { is_expected.to include(is_supported_content: false) }
+      it { is_expected.to include(is_supported_content: 'false') }
     end
 
     context 'when file does not exist' do
       let(:file_path) { 'UNKNOWN.md' }
 
-      it { is_expected.to include(is_supported_content: false) }
+      it { is_expected.to include(is_supported_content: 'false') }
     end
 
     context 'when repository is empty' do
       let(:project) { create(:project_empty_repo) }
 
-      it { is_expected.to include(is_supported_content: false) }
+      it { is_expected.to include(is_supported_content: 'false') }
     end
   end
 end