Return the header object that has the given (string) header
        class name.  Returns None if no such header exists.

def get_header_by_name(self, hdrname):
        '''
        Return the header object that has the given (string) header
        class name.  Returns None if no such header exists.
        '''
        for hdr in self._headers:
            if hdr.__class__.__name__ == hdrname:
                return hdr
        return None