Fix warning for boolean props

[jonahtanjz]

What is the purpose of this pull request?

• Documentation update
• Bug fix
• Feature addition or enhancement
• Code maintenance
• Others, please explain:

Overview of changes:
Resolves #1628.

Add String type to Boolean props with default value of true. This is because HTML attribute value is passed as String.
In this case, false will be rendered as String:

<panel header="..." expandable="false">
  ...
</panel>

In this case, minimized will be passed as Boolean:

<panel header="..." minimized>
  ...
</panel>

Anything you'd like to highlight / discuss:

Should we add String type to all Boolean props that are from HTML attributes? Currently I only see the need for props with default value of true as the only way to set it to false as a HTML attribute is to pass a "false" value.
However, if a user input this,

<panel header="..." minimized="true">
  ...
</panel>

minimized will be rendered as String. Not sure if the above example is the correct usage of HTML in general. Should we leave this to the user such that if they want an attribute to be true, just include the attribute there without any value? The above example of passing a true value will trigger warning unless we include String type for all Boolean props coming from HTML.

Testing instructions:
Follow steps in #1628.

Proposed commit message: (wrap lines at 72 characters)
Fix warning for Boolean props

---

Checklist: ☑️

• Updated the documentation for feature additions and enhancements
• Added tests for bug fixes or features
• Linked all related issues
• No blatantly unrelated changes
• Pinged someone for a review!

[ang-zeyu]

Should we leave this to the user such that if they want an attribute to be true, just include the attribute there without any value?

Agree with this. Going forward we should (have been for while already though) stick with this for new component props. It should be simpler for both the user and implementation wise.

For the previous existing attributes using this behaviour, it is a breaking change, so we can slate that for v4.
In the meantime I think the docs need some updating on true / false versus "just put the property name there" though =X

Should we add String type to all Boolean props that are from HTML attributes?

It should rightly be added for all props using the toBoolean function (a relic from vue v1->v2 migration), but I think its ok to fix-this-as-we-go to minimise possible side effects, the warning from vue is not critical in comparison. But if you're up for investigating each usage let's do that 😀

[jonahtanjz]

Thanks for the review @ang-zeyu

In the meantime I think the docs need some updating on true / false versus "just put the property name there" though

Yup agree. Will open another issue and PR for this :)

It should rightly be added for all props using the toBoolean function (a relic from vue v1->v2 migration), but I think its ok to fix-this-as-we-go to minimise possible side effects, the warning from vue is not critical in comparison. But if you're up for investigating each usage let's do that

Ok got it! Let's go with this 👍

[jonahtanjz]

@ang-zeyu I have updated the PR to include String to all relevant props using toBoolean.

[ang-zeyu]

Lgtm!

[ang-zeyu]

👍