Fix problem with returning empty sets in hal+json.

When fetching the list of studies with HTTP header
'Accept: application/hal+json', the application returned code 
406 'Not acceptable' if the set of studies was empty.

grails-app/controllers/org/transmartproject/rest/StudyController.groovy

@@ -25,10 +25,14 @@
 
 package org.transmartproject.rest
 
-import org.transmartproject.core.ontology.StudiesResource
+import grails.rest.Link
 
 import javax.annotation.Resource
 
+import org.transmartproject.core.ontology.StudiesResource
+import org.transmartproject.core.ontology.Study
+import org.transmartproject.rest.marshallers.ContainerResponseWrapper
+
 class StudyController {
 
     static responseFormats = ['json', 'hal']
@@ -40,7 +44,7 @@ class StudyController {
      *  This will return the list of studies, where each study will be rendered in its short format
     */
     def index() {
-        respond studiesResourceService.studySet
+        respond wrapStudies(studiesResourceService.studySet)
     }
 
     /** GET request on /studies/${id}
@@ -51,4 +55,18 @@ class StudyController {
     def show(String id) {
         respond studiesResourceService.getStudyById(id)
     }
+
+    private def wrapStudies(Object source) {
+
+        new ContainerResponseWrapper(
+                container: source,
+                componentType: Study,
+                links: [
+                        new Link(grails.rest.render.util.AbstractLinkingRenderer.RELATIONSHIP_SELF,
+                                "/studies"
+                        )
+                ]
+        )
+    }
+
 }