bugfix: wip: Set headers in the tail of a list rather than in the head

[oxpa]

It seems that ngx_http_variable_headers_internal function in nginx/src/http/ngx_http_variables.c relies on the fact that the last header values is stored in the last element of a list. Therefore we should avoid changing a random element in the head of a list of headers and either change the last element or add a new element to the list.

Not sure if this is the proper fix. Thanks to @ssdr for pointing out the problem in #2177

I hereby granted the copyright of the changes in this pull request to the authors of this lua-nginx-module project.