Issue: SPR-12352

Issue: SPR-12340

Use explicit flag whether to copy all attributes.

The WebSocketMessageBroker config now allows wrapping the
SubProtocolWebSocketHandler to enable advanced use cases that may
require access to the underlying WebSocketSession.

Issue: SPR-12314

Issue: SPR-12314

Issue: SPR-12301

This change adds a new XhrTransport for the SockJs client
implementation. This transport is based on `UndertowClient`,
Undertow's HTTP client.

Note that this transport can be customized with an OptionMap that is
used by the Xnio worker backing the I/O communications (see
http://xnio.jboss.org).

Implementation tested with undertow 1.0.36, 1.1.0.RC3, 1.2.0.Beta1.

Issue: SPR-12008

This change adds a "Vary: Origin" HTTP response header for /info and
/iframe SockJS endpoints.
This is preventing proxies and browsers from caching a response and
reusing it for an invalid Origin.

Reference: https://groups.google.com/forum/#!topic/sockjs/svsLWRorSis

Issue: SPR-12310

Issue: SPR-12311

TextMessage.toString() does not throw StringIndexOutOfBoundsException for payload with multibyte characters

Issue: SPR-12307

This commit updates the default location of the SockJS' client library.
The previous location is being retired by the project maintainers.

The new default location is backed by several CDN providers:
* https://cdn.jsdelivr.net/sockjs/0.3.4/sockjs.min.js

See sockjs/sockjs-client#198

Issue: SPR-12254

This change uses a ChannelInterceptor (inserted at index 0) to detect
when a DISCONNECT message is precluded from being sent on the
clientInboundChannel. This can happen if another interceptor allows
a runtime exception out from preSend or returns false.

It is crucial for such messages to be processed, so when detected
they're processed still.

Issue: SPR-12218

Introduced addScope convenience method on CustomScopeConfigurer (for use in WebSocket configuration)

Issue: SPR-12249

Issue: SPR-12247

WebSocket clients going away is an expected and common occurance.
Logging at ERROR level on failure to close a connection or on failures
to write data to a WebSocket sessions has a high potential for false
positives with very little to do. This change lowers the log level for
a number of log messages that fit this category.

This should be helped by the effort already spent for 4.1 to ensure
logging at DEBUG level doesn't produce excessive amounts of logging.

Issue: SPR-12155

The WebSocket messaging namespace now exposes configuration options for
custom argument resolvers and return value handlers.

Issue: SPR-12217

This change extends the "websocket" scope to ApplicationContext events
published from StompSubProtocolHandler. This however will only work
with ApplicationEventMulticaster that multicasts events in the same
thread.

Issue: SPR-12172

SessionDisconnectEvent now extends AbstractSubProtocolEvent.

Issue: SPR-12156

Issue: SPR-12215

Issue: SPR-11870

Issue: SPR-12091

ServerEndpointExporter can initialize itself based on a late-provided ServletContext as well (for Boot)

Also allows for direct setting of a ServerContainer and for custom triggering of endpoint registration.

Issue: SPR-12109

Consistent use of BDDMockito rather than standard Mockito.

Replace references to the old RFC 2616 (HTTP 1.1) with references
to the new RFCs 7230 to 7235.

This commit also deprecates:
 - HttpStatus.USE_PROXY
 - HttpStatus.REQUEST_ENTITY_TOO_LARGE in favor of HttpStatus.PAYLOAD_TOO_LARGE
 - HttpStatus.REQUEST_URI_TOO_LONG in favor of HttpStatus.URI_TOO_LONG

Issue: SPR-12067

Before this change the SockJsWebSocketHandler precluded determination
of the sub-protocols by wrapping the actual target handler.

After this change SockJsWebSocketHandler implements SubProtocolCapable
and returns the list of sub-protocols from the wrapped handler.

SockJsClient now implements WebSocketClient directly without extending
AbstractWebSocketClient.

Issue: SPR-12030