Safer header check (#125)

Author: soulgalore

Diff for: lib/headers.js

@@ -47,7 +47,10 @@ module.exports = {
     const maxAgeRegExp = /max-age=(\d+)/;
     let expireSeconds = 0;
 
-    if (responseHeaders['cache-control']) {
+    if (
+      responseHeaders['cache-control'] &&
+      responseHeaders['cache-control'].length > 0
+    ) {
       if (
         responseHeaders['cache-control'][0].indexOf('no-cache') !== -1 ||
         responseHeaders['cache-control'][0].indexOf('no-store') !== -1