This commit adds auto-configuration support for WebSocket-based
messaging. When the user enables WebSocket messaging (typically via
@EnableWebSocket and @EnableWebSocketMessageBroker) and an ObjectMapper
bean exists, a MappingJackson2MessageConverter that uses the
ObjectMapper will be configured. This causes any spring.jackson.*
configuration to affect WebSocket message conversion in the same way
as it affects HTTP message conversion.

Closes gh-2445