AMQP-363: Fix `ignoresThrowable` for AmqpAppender

JIRA: https://jira.springsource.org/browse/AMQP-363

Previously the `AmqpAppender` didn't check `layout.ignoresThrowable`
and added `ThrowableInformation` to the end of log message.

* Add check `layout.ignoresThrowable()` and now it allow to use custom `Layout`, e.g. JSON or XML
* Add test for `org.apache.log4j.xml.XMLLayout`