Implicitly add the ${} around groovyExpression. When cxml contains ${}, it fights with spring's PropertyPlaceholderConfigurer (if enabled); and since what we're interested in is an expression, not a template for producing a string, it's more appropriate to leave it out anyway.
Merge pull request #52 from nlevitt/expression-deciderule

Implicitly add the ${} around groovyExpression. When cxml contains ${}, ...