net: http_server: Add support for generic HTTP1 500 response

In case of internal server errors during HTTP1 request processing,
send 500 Internal Server Error response before shutting down the
connection.

Make sure http1_headers_sent is set whenever sever starts replying, to
avoid duplicate response in case of errors, as that would be protocol
violation.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>