Thanks, Mark, for your attention, but actually I don't see how `ExpressionSource` can help us.
The reason of this change not get expressions at runtime, but get entire Map of uri placeholders at runtime and from Message context.
Our discussion with Gary is around a dilemma to wrap the result of 
`this.uriVariablesExpression.getValue(this.evaluationContext, requestMessage, Map.class); ` to the `ExpressionEvalMap`
or not.