[12.x] Fix escaping of AppendableAttributeValue

[basantashubhu]

I recently ran into a problem where appendable attribute does not escape it's value AppendableAttributeValue cause the function shouldEscapeAttributeValue returns false if it is an object. So, checked the value if it is an instance of AppendableAttributeValue and then get the actual value for escaping.

if ($value instanceof AppendableAttributeValue) {
    return $this->shouldEscapeAttributeValue($escape, $value->value);
}