XML Handler

A number of issues with the XML functions in the state environment.

- Successive "RemoveAttribute" calls result in:
"
Exception thrown: read access violation.
**this** was 0x68.
"
- SaveToString calls once an attribute has been removed and then added throws the same error
- SaveToString calls can duplicate "xmlns=..."

This error can be repeated by including an XML file in the resource folder and using this code:

auto xml_string = pStateEnvironment->LoadResourceString("test");
auto xml_object = pStateEnvironment->ParseXMLString(xml_string);
auto xml_string_back = xml_object->SaveToString(false);

首先，使用 issue 中所示的 XML 资源以及对 LoadResourceString、ParseXMLString、RemoveAttribute 和 SaveToString 的调用来复现报告。跟踪连续删除属性以及删除后再添加的序列化过程，然后验证 SaveToString 不再崩溃，也不会重复生成 xmlns 条目。

由索引模型根据 Issue 内容生成。