Bug 1468830 ignore exceptions when retreiving response headers, r=rpl

MozReview-Commit-ID: Dw4X1q15NHv

--HG--
extra : rebase_source : 155d8afd083f250d91e77f738ce425b7c2e2b06b

toolkit/modules/addons/WebRequest.jsm

@@ -757,8 +757,10 @@ HttpObserverManager = {
         }
 
         if (opts.responseHeaders) {
-          responseHeaders = responseHeaders || new ResponseHeaderChanger(channel);
-          data.responseHeaders = responseHeaders.toArray();
+          try {
+            responseHeaders = responseHeaders || new ResponseHeaderChanger(channel);
+            data.responseHeaders = responseHeaders.toArray();
+          } catch (e) { /* headers may not be available on some redirects */ }
         }
 
         if (opts.requestBody && channel.canModify) {