Fix issue in DefaultUserDestinationResolver

DefaultUserDestinationResolver now uses the session id of
SUBSCRIBE/UNSUBSCRIBE messages rather than looking up all session id's
associated with a user.

Issue: SPR-11325
Added info-level logging for default unit detection

Issue: SPR-11333
Consistent evaluation of empty theme names to default theme name

Issue: SPR-11128
Locale/ThemeChangeInterceptor alignment and javadoc polishing

Issue: SPR-11128
Resolve native header values with @Header

The @Header annotation in spring-messaging now resolves values from the
nested "nativeHeaders" map as well as top-level header map values.

In case of ambiguity (a value that exists in both), the top-level map
value is used and a warning message is printed. This is unlikly in most
cases but can be resolved by prefixing the header value with
"nativeHeadres.myHeader".

Issue: SPR-11326
Fix warnings in HeaderMethodArgumentResolver
Roll back jexcelapi from 2.6.12 to 2.6.3

JExcelAPI 2.6.12 appears to be causing ArrayIndexOutOfBoundsException
with the build. Rolling back to the earlier version for now
General ExcelViewTests code update

Issue: SPR-11334
Polishing
Deprecated AbstractJExcelView since JExcelAPI is an abandoned project

JXL had no release since 2009, and has some serious bugs remaining: e.g. JXL on Unix cannot read template files created on Windows, using the 'latest' JXL 2.6.12 release.
Consider wildcard type without bounds as eligible for fallback match too

Issue: SPR-11250
Upgrade to jexcelapi 2.6.12 and fix test failures

Upgrade to the latest release of jexcelapi and work-around the
ArrayIndexOutOfBounds test exception on *nix machines.

Issue: SPR-11334
Allow HttpHeaders return values for @MVC methods

Allow HttpHeader instances to be returned directly from MVC controller
methods managed by HandlerMethodReturnValueHandler rather than needing
to be wrapped in a ResponseEntity.

Issue: SPR-11129
Polish

Issue: SPR-11129