perf(headers): don't render template string when delete header

CHANGELOG.md

@@ -11,6 +11,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 - Camel Policy: avoid unnecessary closure allocation per request [PR #1584](https://github.com/3scale/APIcast/pull/1584)
 - Logging Policy: construct all templates at init time [PR #1587](https://github.com/3scale/APIcast/pull/1587)
 - Routing Policy: eliminate unnecessary TemplateString allocation per request [PR #1585](https://github.com/3scale/APIcast/pull/1585)
+- Headers Policy: don't render template string when delete header [PR #1586](https://github.com/3scale/APIcast/pull/1586)
 
 ### Fixed
 - Correct FAPI header to `x-fapi-interaction-id` [PR #1557](https://github.com/3scale/APIcast/pull/1557) [THREESCALE-11957](https://issues.redhat.com/browse/THREESCALE-11957)

gateway/src/apicast/policy/headers/headers.lua

@@ -88,7 +88,10 @@ local command_functions = {
 local function run_commands(context, commands, header_type, ...)
   for _, command in ipairs(commands) do
     local command_func = command_functions[header_type][command.op]
-    local value = command.template_string:render(context)
+    local value
+    if command.op ~= 'delete' then
+      value = command.template_string:render(context)
+    end
 
     command_func(command.header, value, ...)
   end