Make TagRecord class safer to use

--HG--
branch : dev

src/Orchard.Web/Modules/Orchard.Tags/Models/TagRecord.cs

@@ -2,6 +2,9 @@
 
 namespace Orchard.Tags.Models {
     public class TagRecord {
+        public TagRecord() {
+            ContentTags = new List<ContentTagRecord>();
+        }
         public virtual int Id { get; set; }
         public virtual string TagName { get; set; }
         public virtual IList<ContentTagRecord> ContentTags { get; set; }