all expressions should be serializable

spring-binding/src/main/java/org/springframework/binding/expression/Expression.java

@@ -15,6 +15,8 @@
  */
 package org.springframework.binding.expression;
 
+import java.io.Serializable;
+
 /**
  * An expression capable of evaluating itself against context objects. Encapsulates the details of a previously parsed
  * expression string. Provides a common abstraction for expression evaluation independent of any language like OGNL or
@@ -22,7 +24,7 @@ package org.springframework.binding.expression;
  * 
  * @author Keith Donald
  */
-public interface Expression {
+public interface Expression extends Serializable {
 
 	/**
 	 * Evaluate this expression in the provided context and return the result of evaluation.