RESTEasyContextMapper requires all context property values to be Strings, which is a bit too restrictive, IMO. If the context mapper is configured to map the property, and it's non-null, then we should just call toString() on the property instead of doing an instanceof String.

https://github.com/jboss-switchyard/components/blob/master/resteasy/src/main/java/org/switchyard/component/resteasy/composer/RESTEasyContextMapper.java#L81