If there is `json`-aware `contentType` header in the message to process
and payload type is `String` or `byte[]` and it isn't equal to the
expected exclusive method argument type (even generic from the `Message<>`),
the conversion should happen only if SpelInvoker is no method.
The regular `InvocableHandlerMethod` takes care about payload conversion
via configured `MessageConverter` in the `PayloadArgumentResolver`
* Fix `MessagingMethodInvokerHelper` to call JSON conversion only when
`this.handlerMethod.spelOnly`
NOTE: We can't do generics conversion right now because it is going
to be a breaking change around
`org.springframework.integration.support.json.JsonObjectMapper` property
in the `MessagingMethodInvokerHelper`.
We can do that only in `5.1` and rely there only on the functionality
from the Jackson Object mapper
* Convert from JSON only in the `invokeExpression()`
* Optimize message recreation only if target param type is `Message`.
Otherwise override just `payload` property of the `ParametersWrapper`
2c3ee7d68c