* GH-2765: Add discardChannel for splitter
Fixes https://github.com/spring-projects/spring-integration/issues/2765
When encountering empty collections, splitter should be able to send
the result to a discard channel.
Currently, when encountering an empty collection,
the splitter ends the flow.
Some use-cases may rely on a custom split function which may returns
empty collections.
These use-cases should be able to define a discard channel
so they can proceed with a possible compensation flow.
* Add `discardChannel` option to the `AbstractMessageSplitter`
* Delegate `discardChannel` population from everywhere it is possible:
DSL, XML, `AbstractMessageSplitter` extension like `FileSplitter` etc.
* Fix `FileSplitterTests` for broken charset
* Document new feature; fix some typos and out-dated code sample
* * Fix `SplitterFactoryBean` for NPE on the `discardChannelName`
propagation
* * Check `this.discardChannel` first
* `Assert.state()` in `doInit()` for mutual exclusiveness
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