Return the first index of the header class hdrclass
        starting at startidx (default=0), or -1 if the
        header class isn't found in the list of headers.

def get_header_index(self, hdrclass, startidx=0):
        '''
        Return the first index of the header class hdrclass
        starting at startidx (default=0), or -1 if the
        header class isn't found in the list of headers.
        '''
        for hdridx in range(startidx, len(self._headers)):
            if isinstance(self._headers[hdridx], hdrclass):
                return hdridx
        return -1