INT-3199: Fix inconsistency for candidate methods

JIRA: https://jira.springsource.org/browse/INT-3199

* Add a `Void` `TypeDescriptor` for methods without parameters
and leave it as `List` for case `canProcessMessageList = true`
* Add sort for `handlerMethods#keySet()` to make `Message`-methods with precedence
* Wrap result of sort to `LinkedHashMap`