Parser erroneously adds `"@_/": true` attr for self-closing stop nodes

[smoores-dev]

• [X] Are you running the latest version?
• [X] Have you included sample input, output, error, and expected output?
• [X] Have you checked if you are using correct configuration?
• [ ] Did you try online tool? (there doesn't seem to be a way to add stop nodes to the online tool)

Description

When the `allowBooleanAttributes` flag is set on `XMLParser`, the parser will interpret the closing `/` in a self-closing stop node as a boolean attribute. This introduces further problems when attempting to build the parsed data back into a string, as it produces invalid XML. E.g., ` Githubissues.

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