Backslash escape line.value.

def encode(cls, line):
        """Backslash escape line.value."""
        if not line.encoded:
            encoding = getattr(line, 'encoding_param', None)
            if encoding and encoding.upper() == cls.base64string:
                line.value = b64encode(line.value).decode('utf-8')
            else:
                line.value = backslashEscape(str_(line.value))
            line.encoded = True