Previously, WebSocketMessagingAutoConfiguration added a single
additional converter. This was a MappingJackson2MessageConverter
configured with the auto-configured ObjectMapper.
AbstractMessageBrokerConfiguration places additional converters before
any of the default converters. This meant that the auto-configuration
had the unwanted side-effect of changing the ordering of the
converters. A MappingJackson2MessageConverter was now first in the
list, whereas, by default, it's last in the list after a
StringMessageConverter and a ByteArrayMessageConverter.

This commit updates WebSocketMessagingAutoConfiguration so that it
switches off the registration of the default converters and registers
a StringMessageConverter, ByteArrayMessageConverter and
MappingJackson2MessageConverter in that order. A test has been
added to verify that the types of these three converters match
the types of the default converters. A second test that verifies
that String responses are converted correctly has also been added
alongside the existing test that verified the behaviour for JSON
responses.

Closes gh-5123