Use more robust Array instance check

Improve robustness of Array type check.

Author: rubendg

lib/test.js

@@ -236,7 +236,7 @@ Test.prototype._assertHeader = function(header, res) {
 
   if (typeof actual === 'undefined') return new Error('expected "' + field + '" header field');
   // This check handles header values that may be a String or single element Array
-  if ((actual instanceof Array && actual.toString() === fieldExpected)
+  if ((Array.isArray(actual) && actual.toString() === fieldExpected)
     || fieldExpected === actual) {
     return;
   }