Previously the default number of retry is equal to the number of node,
but the same node will be tried again according to its weight.
Also ensure the same picker will be used in the whole request,
especially during the retry.
* 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.
