ResourceRegionHttpMessageWriter no longer extends from
EncoderHttpMessageWriter freeing it to pass the correct content type
into the encoder.
Considering that the main benefit of EncoderHttpMessageWriter is to
deal with content type fallback cases, there is nothing to be missed.
Furthermore ResourceRegionHttpMessageWriter is a package private class
that is used internally within ResourceHttpMessageWriter and never
exposed externally as a an actual HttpMessageWriter.
Issue: SPR-15358
c735ffb39b