def get_headers(kwargs):
    """Get a single set of headers from the kwargs dict. A user agent is added
       as it is helpful in most cases.

       Parameters
       ==========
       kwargs: the dictionary of keyword arguments that may contain url
               parameters (format is url_param_<name>
    """
    headers = {'User-Agent': 'Mozilla/5.0'}
    for key, value in kwargs.items():
        if key.startswith('header_'):
            name = key.replace('header_', '', 1)
            if value is not None:
                headers[name] = value
            elif value is None and name in headers:
                del headers[name]
    return headers
