As of Gradle Kotlin plugin 1.1.60,
project.sourceSets.main.kotlin.srcDirs contains
Java source directories in addition to Kotlin
ones. This commit filter out Java sources to
retain only Kotlin ones.

Issue: SPR-16268

* pr/1615:
  Fix exception message

initialDelayString is parsed into long, not integer.

Closes gh-1615

Issue: SPR-16237

This commit fixes a bug where an IllegalStateException was
thrown if the minPort and maxPort values supplied to
SocketUtils.findAvailableTcpPort(int, int) were identical.

Reduce access on user in SimpleBrokerMessageHandler.handleMessageInternal

Issue: SPR-16264

From #9a894a we began using the content-type from the ResponseEntity
but cross-checking it still against the requested content type.

Arguably there isn't any purpose in cross-checking. The only possible
outcomes are: a) it's compatible or b) it's not, which would result in
406 (NOT_ACCEPTABLE). As we've been given explicitly the media type to
use, it makes little sense to send 406, ignoring the wish to use that
content type.

Issue: SPR-16251

The request URI returned from HttpServerRequest.uri() typically
contains contains an absolute path but could also contain an
absolute URI. This commit adds handling for the latter, effectively
taking only the absolute path portion.

Issue: SPR-16243

Issue: SPR-16244

apiVersion and languageVersion options are set to 1.1
on production code in order to avoid incompatibilities
with Kotlin 1.1 based projects or libraries.

Issue: SPR-16239

Issue: SPR-16240

ResponseStatusExceptionHandler lets the error through if it can't
change the status while HttpWebHandlerAdapter logs a more helpful
message (including status code) but without a full stack trace.

Issue: SPR-16231

Issue: SPR-16241

- Add "Processing" section (also replaces Advanced Customizations)
- Add information on out-of-the-box behavior
- Add more deails on @CrossOririn default configuratio
- Add cross-references between Spring MVC and WebFlux
- Polish

1. setComplete checks the isCommitted flag to avoid an unnecessary
debug message. This method is meant to be safe to call many times.

2. setStatusCode lowers log message to TRACE, since the return value
communicates the outcome it's arguably much less critical.

3. Add comment and test case for ResponseStatusExceptionHandler.
A ResponseStatusException is clearly meant to be handled by this
handler so don't let it pass through even if the respones is
committed.

Issue: SPR-16231

Issue: SPR-16225

Issue: SPR-16194

Issue: SPR-16230

Issue: SPR-16229

Before this commit, the return value from write was interpreted as the
data being fully written and ready to be released via releaseData().

This is not true for WebSocketSession implementations where a true
return value simply means the message was sent with the full payload
but releas is not appropriate until a send confirmation.

Technically not an issue since WebSocketSession's extending this do
not use pooled buffers. Nevertheless this commit refines the semantics
of write, removes the releaseData() method, and makes sub-classes
responsible for releasing the buffer when fully written (and they
know best when that is). As a bonus currentData is now private.

Issue: SPR-16207