Improve configs for `.rdf` files

* Make Apache serve `.rdf` files with the charset parameter set to
  `UTF-8`.

* Add configs for the cases¹ when the media type for `.rdf` files
  is set to `application/rdf+xml`, namely, ensure that Apache serves
  files marked with that media type compressed and with far future
  expires headers.

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¹ https://github.com/h5bp/server-configs-apache/commit/db6932740a90a36cbbf8b38627fc034d595471c0