How to use the gunicorn.util.to_bytestring function in gunicorn

def test_to_bytestring():
    assert util.to_bytestring('test_str', 'ascii') == b'test_str'
    assert util.to_bytestring('test_str®') == b'test_str\xc2\xae'
    assert util.to_bytestring(b'byte_test_str') == b'byte_test_str'
    with pytest.raises(TypeError) as exc_info:
        util.to_bytestring(100)
    msg = '100 is not a string'
    assert msg in str(exc_info.value)

response.headers.append(('foo', 'häder'))
    response.send_headers()

    # set a-breve header value - unicode, non-latin-1 fails
    response = Response(mocked_request, mocked_socket, None)
    response.headers.append(('apple', 'măr'))
    with pytest.raises(UnicodeEncodeError):
        response.send_headers()

    # build our own header_str to compare against
    tosend = response.default_headers()
    tosend.extend(["%s: %s\r\n" % (k, v) for k, v in response.headers])
    header_str = "%s\r\n" % "".join(tosend)

    with pytest.raises(UnicodeEncodeError):
        mocked_socket.sendall(util.to_bytestring(header_str, "ascii"))

def send_headers(self):
        if self.headers_sent:
            return
        tosend = self.default_headers()
        tosend.extend(["%s: %s\r\n" % (k, v) for k, v in self.headers])

        header_str = "%s\r\n" % "".join(tosend)
        util.write(self.sock, util.to_bytestring(header_str, "latin-1"))
        self.headers_sent = True

def send_headers(self):
        if self.headers_sent:
            return
        tosend = self.default_headers()
        tosend.extend(["%s: %s\r\n" % (k, v) for k, v in self.headers])

        header_str = "%s\r\n" % "".join(tosend)
        util.write(self.sock, util.to_bytestring(header_str, "ascii"))
        self.headers_sent = True

def send_headers(self):
        if self.headers_sent:
            return
        tosend = self.default_headers()
        tosend.extend(["%s: %s\r\n" % (k, v) for k, v in self.headers])

        header_str = "%s\r\n" % "".join(tosend)
        util.write(self.sock, util.to_bytestring(header_str, "latin-1"))
        self.headers_sent = True

def send_headers(self):
        if self.headers_sent:
            return
        tosend = self.default_headers()
        tosend.extend(["%s: %s\r\n" % (k, v) for k, v in self.headers])

        header_str = "%s\r\n" % "".join(tosend)
        util.write(self.sock, util.to_bytestring(header_str, "ascii"))
        self.headers_sent = True

def send_headers(self):
        if self.headers_sent:
            return
        tosend = self.default_headers()
        tosend.extend(["%s: %s\r\n" % (k, v) for k, v in self.headers])

        header_str = "%s\r\n" % "".join(tosend)
        util.write(self.sock, util.to_bytestring(header_str, "ascii"))
        self.headers_sent = True