bugfix: body sometimes empty

[bjuergens]

sometimes the body stays empty.

this can be fixed with

'''

  EXTRA_DIRECTIVES_SERVER: |
    set $$req_body "";
    # workaround for nginx-feature, where $$request_body is sometimes empty
    rewrite_by_lua_block {
        ngx.var.req_body = ngx.req.get_body_data();
    }

'''

source:

• https://gist.github.com/morhekil/1ff0e902ed4de2adcb7a?permalink_comment_id=2658146#gistcomment-2658146
• https://forum.nginx.org/read.php?2,289350,289356

--> todo: include this in the default template for the server block.