INT-2433: Get Rid of MessageProcessors wrapping

Previously, provided `MessageProcessor` to create a `MessageHandler`
for endpoint was wrapped to `MethodInvokingMessageProcessor`,
e.g. `GroovyScriptExecutingMessageProcessor`.

Check the type of provided object in the constructor
of `MethodInvoking*` strategies and use it directly if it is `MessageProcessor`

JIRA: https://jira.springsource.org/browse/INT-2433