INT-4540: Use MessageHeaders in GenericHandler (#2589)

* INT-4540: Use MessageHeaders in GenericHandler

JIRA: https://jira.spring.io/browse/INT-4540

For better end-user experience use a `MessageHeaders` in the
`GenericHandler` instead of plain `Map<String, Object>`

* * Fix `IntegrationFlowEventsTests`

src/reference/asciidoc/dsl.adoc

@@ -476,7 +476,7 @@ Similar lambda configurations are provided for the `resequence()` EIP method.
 The `.handle()` EIP method's goal is to invoke any `MessageHandler` implementation or any method on some POJO.
 Another option is to define an "`activity`" by using lambda expressions.
 Consequently, we introduced a generic `GenericHandler<P>` functional interface.
-Its `handle` method requires two arguments: `P payload` and `Map<String, Object> headers`.
+Its `handle` method requires two arguments: `P payload` and `MessageHeaders headers` (starting with version 5.1).
 Having that, we can define a flow as follows:
 
 ====

src/reference/asciidoc/whats-new.adoc

@@ -41,6 +41,8 @@ This avoid confusion with `log()` which is treated as a one-way end flow compone
 
 A generated bean name for any `NamedComponent` within an integration flow is now based on the component type for better readability from visual tools, logs analyzers and metrics collectors.
 
+The `GenericHandler.handle()` now excepts a `MessageHeaders` type for the second argument.
+
 [[x5.1-dispatcher-exceptions]]
 ==== Dispatcher Exceptions