Add accept_language method to request to extract HTTP_ACCEPT_LANGUAGE

HTTP_ACCEPT_LANGUAGE header (RFC 2616) allows browser to pass the user's language preferences to the server. This behaves effectively the same as HTTP_ACCEPT_ENCODING. The added accept_language method is based on accept_encoding. Likewise, the spec for the method is very similar to that for accept_encoding.

Moved duplicate code into utility method parse_http_accept_header as per raggi request - passes tests