mediaType: clean up charset handle

[jknack]

• remove toContentTypeHeader(Charset) - Charset must be defined as content-type parameter or use default one
• remove UTF-8 from application/json
• Remove Context.setResponseType(MediaType, Charset), Context.setResponseType(MediaType) is enough
• Remove Context.responseWriter(MediaType, Charset), Context.responseWriter(MediaType) is enough