The `FunctionConfiguration.sanitize()` use a `rawtypes` trying to hide
generics routines.
Spring Integration 6.4 has introduced a `BaseMessageBuilder` which does not work
with `rawtypes`.
* Fix `FunctionConfiguration.sanitize()` to expose a `<P>` generic argument
to satisfy a new `MessageBuilder` byte code, plus to meet all the expectations
of this method consumers
* In addition perform some code clean to optimize a usage of `MessageBuilder`:
- `setHeader()` does set the value overriding existing one
- `setHeader()` removes the entry if provided value is `null`
- This way we just can go ahead with `MessageBuilder` method chain avoiding extra conditions
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