'Renders the headers for this request field.'

def render_headers(self):

    lines = []
    sort_keys = ['Content-Disposition', 'Content-Type', 'Content-Location']
    for sort_key in sort_keys:
        if self.headers.get(sort_key, False):
            lines.append(('%s:    %s' % (sort_key, self.headers[sort_key])))
    for (header_name, header_value) in self.headers.items():
        if (header_name not in sort_keys):
            if header_value:
                lines.append(('%s:    %s' % (header_name, header_value)))
    lines.append('\r\n')
    return '\r\n'.join(lines)
