code: public MediaType withCharset(Charset charset) {
      checkNotNull(charset);
      return withParameter(CHARSET_ATTRIBUTE, charset.name());
    }
docs: Returns a new instance with the same type and subtype as this instance, with the
  {@code charset} parameter set to the {@link Charset#name name} of the given charset. Only one
  {@code charset} parameter will be present on the new instance regardless of the number set on
  this one.
  
  <p>If a charset must be specified that is not supported on this JVM (and thus is not
  representable as a {@link Charset} instance, use {@link #withParameter}.