nghttp2: updated to 1.31.0

nghttp2 v1.31.0:
lib: Add nghttp2_session_set_user_data() public API function
src: Define nghttp2_inet_pton wrapper to avoid inet_pton macro
nghttpx: Close listening socket on graceful shutdown
nghttpx: Add an option to accept expired client certificate
nghttpx: Add mruby tls_client_not_before, and tls_client_not_after
nghttpx: Fix potential memory leak

(adam)