config: whitelist a few more directives for script variables

When configuring downstream caching if you have an origin with multiple caches in front of it you might want to choose how you handle requests in response to which cache you're getting them from.  Whitelist DownstreamCachePurgeLocationPrefix, DownstreamCachePurgeMethod, and DownstreamCacheRewrittenPercentageThreshold for scripting so this is possible.

src/ngx_rewrite_options.cc

@@ -276,10 +276,15 @@ const char* NgxRewriteOptions::ParseAndSetOptions(
 
   ScriptLine* script_line;
   script_line = NULL;
-  // Only allow script variable support for LoadFromFile for now.
   // Note that LoadFromFile should not be scriptable on wildcard hosts,
   // as browsers might be able to manipulate its natural use-case: $http_host.
-  if (!StringCaseStartsWith(directive, "LoadFromFile")) {
+  if (!StringCaseStartsWith(directive, "LoadFromFile") &&
+      !StringCaseEqual(directive, "EnableFilters") &&
+      !StringCaseEqual(directive, "DisableFilters") &&
+      !StringCaseEqual(directive, "DownstreamCachePurgeLocationPrefix") &&
+      !StringCaseEqual(directive, "DownstreamCachePurgeMethod") &&
+      !StringCaseEqual(directive,
+                       "DownstreamCacheRewrittenPercentageThreshold")) {
     compile_scripts = false;
   }