Automatic Content-Type for FormData uploads

When data passed to axios is of type FormData we have to let the browser
create the Content-Type header so that the boundaries will get right
etc.

Usage:

```js
var data = new FormData();
data.append('field', 'some string');
data.append('file', someFile);

var opts = {
  transformRequest: function(data) { return data; }
};

axios.post('/fileupload', data, opts);

```

lib/adapters/xhr.js

@@ -21,6 +21,10 @@ module.exports = function xhrAdapter(resolve, reject, config) {
     config.headers || {}
   );
 
+  if (utils.isFormData(data)) {
+    delete headers['Content-Type']; // Let the browser set it
+  }
+
   // Create the request
   var request = new(XMLHttpRequest || ActiveXObject)('Microsoft.XMLHTTP');
   request.open(config.method, buildUrl(config.url, config.params), true);