Return the first header object that is of
        class hdrclass, or None if the header class isn't
        found.

def get_header(self, hdrclass, returnval=None):
        '''
        Return the first header object that is of
        class hdrclass, or None if the header class isn't
        found.
        '''
        if isinstance(hdrclass, str):
            return self.get_header_by_name(hdrclass)

        for hdr in self._headers:
            if isinstance(hdr, hdrclass):
                return hdr
        return returnval