lib: fix templating being overzealous

It was not really restricted to HTML files.

lib/template.js

@@ -19,7 +19,7 @@ module.exports = function(base, substitutions) {
   return function(req, res, next) {
     // Handle only GET/HEAD files
     if ((req.method !== 'GET' && req.method !== 'HEAD') ||
-        req.path.slice(1) in (onlyFor || []) === -1) {
+        (onlyFor || []).indexOf(req.path.slice(1)) === -1) {
       return next();
     }
     var write = res.write;