'Add all custom Headers() if they exist'

def add_headers(self, req, headers=Headers()):

    for (h, v) in self.settings.header_list:
        req.add_header(h, v)
    for (h, v) in headers.iteritems():
        req.add_header(h, v)
    if (self.settings.rand_user_agent is True):
        req.add_header('User-Agent', get_random_user_agent())
    return req
