Allow FieldOverride to be constructed with a ColumnExp.

src/java/com/samskivert/depot/clause/FieldOverride.java

@@ -49,4 +49,9 @@ public class FieldOverride extends FieldDefinition
     {
         super(field, override);
     }
+
+    public FieldOverride (ColumnExp field, SQLExpression override)
+    {
+        super(field, override);
+    }
 }