treewide: improve template expansion

also: support *.toml as extra configuration dictionaries

angie/snip/http-response-headers.j2

@@ -1,6 +1,5 @@
 ## sourced by autoconf/http-response-headers.conf
 ## add response headers
-{%- set resp_hdr_dict = j2cfg.response_headers or {} -%}
-{%- for h, v in resp_hdr_dict|dictsort %}
-add_header  {{ h | ngx_esc }}  {{ v | ngx_esc }};
+{%- for h, v in j2cfg.response_headers | http_header | dictsort %}
+add_header  {{ h }}  {{ v | ngx_esc }};
 {%- endfor %}