Attributes listed under isBooleanAttr are altered under v-pre attributed tags

[xmakina]

Version

2.6.10

Reproduction link

https://jsfiddle.net/fp1omdaw/1/

Steps to reproduce

In a v-pre tag or child, use an attribute listed under the isBooleanAttr list in src\platforms\web\util\attrs.js

What is expected?

The attribute value is unchanged

What is actually happening?

The attribute value is changed, e.g. open has the value to set to open

---

When working with MathML, we need to use the open attribute