def __init__(self, url, fields, headers=None, mock_http=False):
self.url = url<EOL>self.fields = fields<EOL>self.headers = headers or {}<EOL>self.mock_http = mock_http<EOL>
Initialize POST data.

            Field values can be strings or files; files
            are expected to have a read() method and SHOULD have a 'name'
            attribute (i.e. look like handles returned by 'open()').

            @param url: the URL to POST to.
            @param fields: sequence of (name, value) tuples.
            @param headers: dict of additional headers.