added an overloaded version of toObservable() that takes the body type as an argument so that the body of the message can be extracted and processed as an Observable directly for cases where the user knows the type of the payload in a message

git-svn-id: https://svn.apache.org/repos/asf/camel/trunk@1452197 13f79535-47bb-0310-9956-ffa450edef68