Log Message:
nghttp2: updated to 1.40.0

nghttp2 v1.40.0
lib: Add nghttp2_check_authority as public API (GH-1413)
lib: Fix the bug that stream is closed with wrong error code (GH-1408)
lib: Faster huffman encoding and decoding (GH-1405)
build: Avoid filename collision of static and dynamic lib (Patch from William A \ 
Rowe Jr) (GH-1394)
build: Add new flag ENABLE_STATIC_CRT for Windows (Patch from William A Rowe Jr) \ 
(GH-1393)
build: cmake: Support building nghttpx with systemd (Patch from Andrew Penkrat) \ 
(GH-1377)
third-party: Update neverbleed to fix memory leak
nghttpx: Fix bug that mruby is incorrectly shared between backends (GH-1392)
nghttpx: Reconnect h1 backend if it lost connection before sending headers
nghttpx: Returns 408 if backend timed out before sending headers
nghttpx: Fix request stall (GH-1378)