CharSequenceEncoder now supports all MIME types, however since encoding
Flux<String> can overlap with other encoders (e.g. SSE) there are now
two ways to create a CharSequenceEncoder -- with support for text/plain
only or with support for any MIME type.
In WebFlux configuration we insert one CharSequenceEncoder for
text/plain (as we have so far) and a second instance with support for
any MIME type at the very end.
Issue: SPR-15374
3d68c496f1