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);

```