Add shouldPersistHeaders = true to GraphiQL options

This option allows GraphiQL to persist custom-defined HTTP headers
to local storage. This avoid developers to type those headers again each
time the page is refreshed.

Closes gh-257

spring-graphql/src/main/resources/graphiql/index.html

@@ -40,7 +40,8 @@
         React.createElement(GraphiQL, {
             fetcher: gqlFetcher,
             defaultVariableEditorOpen: true,
-            headerEditorEnabled: true
+            headerEditorEnabled: true,
+            shouldPersistHeaders: true
         }),
         document.getElementById('graphiql'),
     );