Merge pull request #195 from HermanSchoenfeld/master

Mono-compatability bug fix

Author: chester89

src/FluentNHibernate/MappingModel/Collections/LayeredColumns.cs

@@ -21,6 +21,9 @@ public IEnumerable<ColumnMapping> Columns
 
                 foreach (var value in values)
                 {
+					if (value == null)
+						yield break;
+
                     yield return value;
                 }
             }