Honor Accept-Charset when returning text based content

The Accept-Charset can be honored when returning content to a client. So if Accept-Charset: shift-jis;q=0.8, *;q=0.2 is used, then shift-jis should be used as the highest priority encoding for JAXB, String, etc. content instead of defaulting straight to UTF-8. While the JAX-RS spec does not currently dictate this, we should add it.

if a user sets the content-type on the Response, should not change that. If a Content-Type is not specified (i.e. taking some automatic values via @Produces or combination of Accept header), then add a charset.