'Serialize as a header for an HTTPAuth request.'

def to_header(self, realm=''):

    oauth_params = ((k, v) for (k, v) in self.items() if k.startswith('oauth_'))
    stringy_params = ((k, escape(str(v))) for (k, v) in oauth_params)
    header_params = (('%s="%s"' % (k, v)) for (k, v) in stringy_params)
    params_header = ',    '.join(header_params)
    auth_header = ('OAuth    realm="%s"' % realm)
    if params_header:
        auth_header = ('%s,    %s' % (auth_header, params_header))
    return {'Authorization': auth_header}
