Extract FakeRequest.passthrough()

Anytime send is called, the arguments are saved in the instance. They can subsequently be used in the `.passthrough()` method.

src/index.ts

@@ -88,6 +88,7 @@ function interceptor(ctx) {
 
   // extend
   FakeRequest.prototype.send = function send() {
+    this.sendArguments = arguments;
     if (!ctx.pretender.running) {
       throw new Error('You shut down a Pretender instance while there was a pending request. ' +
             'That request just tried to complete. Check to see if you accidentally shut down ' +
@@ -97,13 +98,22 @@ function interceptor(ctx) {
     FakeXMLHttpRequest.prototype.send.apply(this, arguments);
 
     if (ctx.pretender.checkPassthrough(this)) {
-      var xhr = createPassthrough(this);
-      xhr.send.apply(xhr, arguments);
+      this.passthrough();
     } else {
       ctx.pretender.handleRequest(this);
     }
   };
 
+  FakeRequest.prototype.passthrough = function passthrough() {
+    if (!this.sendArguments) {
+      throw new Error('You attempted to passthrough a FakeRequest that was never sent. ' +
+            'Call `.send()` on the original request first');
+    }
+    var xhr = createPassthrough(this);
+    xhr.send.apply(xhr, this.sendArguments);
+    return xhr;
+  };
+
 
   function createPassthrough(fakeXHR) {
     // event types to handle on the xhr

test/passthrough_test.js

@@ -333,4 +333,21 @@ describe('passthrough requests', function(config) {
 
     xhr.send();
   });
+
+  it('allows a passthrough on an unhandledRequest via request.passthrough()', function(assert) {
+    var done = assert.async();
+
+    this.pretender.unhandledRequest = function(_verb, _path, request) {
+      request.passthrough();
+    };
+
+    $.ajax({
+      url: '/some/path',
+      error: function(xhr) {
+        assert.equal(xhr.status, 404);
+        done();
+      }
+    });
+  });
+
 });