fix(search): highlights accepts string parameter (#628)

Author: weakish

src/search.js

@@ -162,7 +162,7 @@ module.exports = function(AV) {
       highlights: function(highlights) {
         var objects;
         if (highlights && _.isString(highlights)) {
-          objects = arguments;
+          objects = _.toArray(arguments);
         } else {
           objects = highlights;
         }