add missing guard
remove `value_is_const`
remove `name_is_token`
nit-pick hpack impl. for simplicity (merge branch 'kazuho/refactor/hpack')
Merge branch 'master' into kazuho/expr/hpack-using-index

Conflicts:
	lib/http2/hpack.c
shrink the size of the table to max-1, before adding a new entry