* bugfix: to get the HTTP request header by `ngx.var.*`, we need to convert
the name to lower case with dashes replaced by underscores.
more information:
http://nginx.org/en/docs/http/ngx_http_core_module.html#var_http_
* change: the key should be a string object when fetching a value from `ctx.var`.
* bug(make lint): If the code style is bad, do exit with code 1.
