Fix typo
tutorial-server.rst: language

I went through and fixed the English and grammar somwhat
Merge branch 'alagoutte-misc'
Merge branch 'server-tutorial' of https://github.com/bagder/nghttp2 into bagder-server-tutorial
Merge branch 'bagder-server-tutorial'
nghttpx: Issue RST_STREAM instead of returning NGHTTP2_ERR_TEMPORAL_CALLBACKFAILURE

NGHTTP2_ERR_TEMPORAL_CALLBACKFAILURE is not supported in
on_frame_recv_callback.
nghttp2_check_header_value: Disallow 0x00