Ignore null headers.

Fixes #180

index.js

@@ -525,11 +525,12 @@ function removeMatchingHeaders(regex, headers) {
   var lastValue;
   for (var header in headers) {
     if (regex.test(header)) {
-      lastValue = headers[header].toString().trim();
+      lastValue = headers[header];
       delete headers[header];
     }
   }
-  return lastValue;
+  return (lastValue === null || typeof lastValue === "undefined") ?
+    undefined : String(lastValue).trim();
 }
 
 function createErrorType(code, defaultMessage) {

test/test.js

@@ -1320,6 +1320,32 @@ describe("follow-redirects", function () {
   });
 
   describe("when the client passes an Authorization header", function () {
+    it("ignores it when null", function () {
+      app.get("/a", redirectsTo(302, "http://localhost:3600/b"));
+      app.get("/b", function (req, res) {
+        res.end(JSON.stringify(req.headers));
+      });
+
+      var opts = url.parse("http://127.0.0.1:3600/a");
+      opts.headers = {
+        host: "localhost",
+        authorization: null,
+      };
+
+      return server.start(app)
+        .then(asPromise(function (resolve, reject) {
+          http.get(opts, resolve).on("error", reject);
+        }))
+        .then(asPromise(function (resolve, reject, res) {
+          res.pipe(concat({ encoding: "string" }, resolve)).on("error", reject);
+        }))
+        .then(function (str) {
+          var body = JSON.parse(str);
+          assert.equal(body.host, "localhost:3600");
+          assert.equal(body.authorization, undefined);
+        });
+    });
+
     it("keeps the header when redirected to the same host", function () {
       app.get("/a", redirectsTo(302, "/b"));
       app.get("/b", function (req, res) {