Ensure type-level Javadoc in every class, comply with guidelines for
80 char on Javadoc, and minor polish.

This commit wraps up the previous commits:

 - It uses HttpMessageConverter in the web.reactive.server package instead of Encoder/Decoder.
 - It introduces tests for the Resource @ResponseBodies.

This commit replaces use of the existing ServletException-based
exceptions with the new ones from ~.web.server.