This method implements a multi-field form submission via a POST, using the ‘fields’ object as a set of name:value pairs to pass as the form fields.   It simply delegates to doPostRequest for the actual processing; the only functionality is serializing the fields.
Note that this method’s parameter ordering does NOT correspond to doPostRequest’s.
Parameters
| url | the url to POST to | 
| fields | the form fields to POST | 
| handler | the callback function to send results to |