Headers policy: add more variables to liquid

[eloycoto]

Discussed with a user,

Headers liquid policy does not have the request schema (http|htts) or the request port.

APIcast/gateway/src/apicast/policy/ngx_variable.lua

Lines 5 to 12 in 981de2f

	local function context_values()
	return {
	uri = ngx.var.uri,
	host = ngx.var.host,
	remote_addr = ngx.var.remote_addr,
	headers = ngx.req.get_headers(),
	http_method = ngx.req.get_method(),
	}