CSW / Test interface does not load samples (#4378)

```
lib.js?v=322d05a25e1afb2d5f2af1a6c9e480a7066e4bf7:126 SyntaxError: Unexpected token < in JSON at position 0
    at JSON.parse (<anonymous>)
    at tc (lib.js?v=322d05a25e1afb2d5f2af1a6c9e480a7066e4bf7:29)
    at $b (lib.js?v=322d05a25e1afb2d5f2af1a6c9e480a7066e4bf7:101)
    at lib.js?v=322d05a25e1afb2d5f2af1a6c9e480a7066e4bf7:101
    at p (lib.js?v=322d05a25e1afb2d5f2af1a6c9e480a7066e4bf7:19)
    at cd (lib.js?v=322d05a25e1afb2d5f2af1a6c9e480a7066e4bf7:101)
    at c (lib.js?v=322d05a25e1afb2d5f2af1a6c9e480a7066e4bf7:103)
    at lib.js?v=322d05a25e1afb2d5f2af1a6c9e480a7066e4bf7:139
    at m.$eval (lib.js?v=322d05a25e1afb2d5f2af1a6c9e480a7066e4bf7:153)
    at m.$digest (lib.js?v=322d05a25e1afb2d5f2af1a6c9e480a7066e4
```
Probably related to Spring MVC update ?

Author: fxprunayre

web-ui/src/main/resources/catalog/js/admin/CSWTestController.js

@@ -65,7 +65,9 @@
 
       $scope.$watch('currentTestId', function() {
         if ($scope.currentTestId !== null) {
-          $http.get('../../xml/csw/test/' + $scope.currentTestId + '.xml')
+          $http.get('../../xml/csw/test/' + $scope.currentTestId + '.xml', {headers: {
+              Accept: 'application/xml'
+            }})
               .success(function(data) {
                 $scope.currentTest = data;
                 $scope.runCSWRequest();