Remove backslash escaping from line.value.

def decode(cls, line):
        """Remove backslash escaping from line.value."""
        if line.encoded:
            encoding = getattr(line, 'encoding_param', None)
            if encoding and encoding.upper() == cls.base64string:
                line.value = b64decode(line.value)
            else:
                line.value = stringToTextValues(line.value)[0]
            line.encoded = False