Merge pull request #289 from steveluscher/patch-1

Guard against `xhr.getAllResponseHeaders()` being `null`

fetch.js

@@ -333,7 +333,7 @@
 
   function headers(xhr) {
     var head = new Headers()
-    var pairs = xhr.getAllResponseHeaders().trim().split('\n')
+    var pairs = (xhr.getAllResponseHeaders() || '').trim().split('\n')
     pairs.forEach(function(header) {
       var split = header.trim().split(':')
       var key = split.shift().trim()