It turns out that `spring-ws-core` brings too much WS stuff which
triggers and auto-configuration to be enabled in Spring Boot app when
we don't expect it.
* The `UnmarshallingTransformer` is already aware of `MimeMessage` and
`MarshallingUtils` being optional, so just remove redundant `public`
modifier for `maybeUnmarshalMimeMessage()` method
* Load `ServletContextResource` class in the `XsltPayloadTransformer`
on demand making such a dependency as optional
e9591c6fdf