fix #6864: don't set ignoredElements' attributes to their default values

[imyzf]

Fix #6864

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
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Does this PR introduce a breaking change? (check one)

• Yes
• No

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• It's submitted to the dev branch for v2.x (or to a previous version branch), not the master branch
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• New/updated tests are included

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Other information:

[yyx990803]

Thanks for the PR. A few issues:

1. In 2.5 ignoredElements also support regular expressions.
2. Placing this check in setAttr can be too expensive. Ideally we don't want to affect performance of the entire framework for such a niche use case.

[imyzf]

@yyx990803 Thanks for your suggestions. I have a new idea:

1. Add a flag isIgnoredElement to VNode
2. Judge and set isIgnoredElement in createElm
3. In setAttr, strip the default attribution process if isIgnoredElement is true

Would this be better?

[yyx990803]

Thanks a lot for the PR. After re-consideration, I think the easier way to deal with this is simply checking if the element could be a custom one when setting the attribute. This avoids the overhead of adding flags to the vnode.