Support XMLHttpRequest withCredentials property

There are flags on the XHR that are important to set correctly in certain
contexts. The 'withCredentials' property used by CORS is one example. As
the context is essential, there is no good default value, thus we need to
make it open to configuration. Rather than explicitly support each property,
the xhr client now supports `request.mixin` which will copy all its
properties directly on to the raw XHR object.

Issue: cujojs#40

client/xhr.js

@@ -77,6 +77,16 @@
 				client = response.raw = new XMLHttpRequest();
 				client.open(method, url, true);
 
+				if (request.mixin) {
+					Object.keys(request.mixin).forEach(function (prop) {
+						// make sure the property already exists as
+						// IE 6 will blow up if we add a new prop
+						if (request.mixin.hasOwnProperty(prop) && prop in client) {
+							client[prop] = request.mixin[prop];
+						}
+					});
+				}
+
 				headers = request.headers;
 				for (headerName in headers) {
 					/*jshint forin:false */

docs/clients.md

@@ -57,6 +57,12 @@ The default client for browsers.  The XHR client utilizes the XMLHttpRequest obj
   <td>window.XMLHttpRequest</td>
   <td>The XMLHttpRequest instance to use</td>
 </tr>
+<tr>
+  <td>request.mixin</td>
+  <td>optional</td>
+  <td><em>none</em></td>
+  <td>Additional properties to mix into the XHR object</td>
+</tr>
 </table>
 
 **Know limitations**

test/client/xhr-test-browser.js

@@ -96,6 +96,17 @@
 					refute(request.canceled);
 				}).otherwise(fail);
 			},
+			'should mixin additional properties': {
+				requiresSupportFor: { timeout: XMLHttpRequest && 'timeout' in new XMLHttpRequest() },
+				'': function () {
+					var request = { path: '/', mixin: { timeout: 1000, foo: 'bar' } };
+					return client(request).then(function (response) {
+						var xhr = response.raw;
+						assert.equals(xhr.timeout, 1000);
+						refute.equals(xhr.foo, 'bar');
+					}).otherwise(fail);
+				}
+			},
 			'//should abort the request if canceled': function (done) {
 				// TODO find an endpoint that takes a bit to respond, cached files may return synchronously
 				// this test misbehavies in IE6, the response is recieved before the request can cancel