add headers to templated html response

[lalyos]

With this change not only query params but the headers are also available in templated html response.

There are at least 2 use cases which would be covered:

• could be used as an upstream for oauth2-proxy
• could be used for various webhooks, which expect a json response with some header/query values

$ BASKET=getuser
$ URL=https://rbaskets.in

$ T=$(curl -s ${URL}/api/baskets/${BASKET} -d '' | jq .token -r )

$ curl -X PUT -H "Authorization: $T" ${URL}/api/baskets/${BASKET}/responses/GET \
  -d '{"status":200,"headers":{},"body":"user: {{ index .headers \"X-Forwarded-Preferred-Username\" }}\nemail: {{ index .headers \"X-Forwarded-Email\" }}","is_template":true}'

## test
$ curl -H "X-Forwarded-Preferred-Username: fake" -H "X-Forwarded-Email: fake@ema.il" ${URL}/${BASKET}

user: [fake]
email: [fake@ema.il]