treewide: improve template expansion

also: support *.toml as extra configuration dictionaries

angie/snip/proxy-request-headers.j2

@@ -1,6 +1,5 @@
 ## sourced by conf/proxy-http/headers.conf
 ## set/remove request headers
-{%- set req_hdr_dict = j2cfg.request_headers or {} -%}
-{%- for h, v in req_hdr_dict|dictsort %}
-proxy_set_header  {{ h | ngx_esc }}  {{ v | ngx_esc }};
+{%- for h, v in j2cfg.request_headers | http_header | dictsort %}
+proxy_set_header  {{ h }}  {{ v | ngx_esc }};
 {%- endfor %}