Added check to passes/web-outgoing.js to make sure the header being s…

…et is not undefined, which should be the only falsey value that could accidently show up and break that call. This fixes windows NTLM auth issues behind http-proxy.

lib/http-proxy/passes/web-outgoing.js

@@ -82,7 +82,9 @@ var redirectRegex = /^30(1|2|7|8)$/;
    */
   function writeHeaders(req, res, proxyRes) {
     Object.keys(proxyRes.headers).forEach(function(key) {
-      res.setHeader(key, proxyRes.headers[key]);
+      if(proxyRes.headers[key] != undefined){
+        res.setHeader(key, proxyRes.headers[key]);
+      }
     });
   },