Collapse widget - default "open all"

• I have searched the issues of this repository and believe that this is not a duplicate.

What problem does this feature solve?

Collapse widget currently allows only one item to be default open. This means most of the content is hidden on load. Widget should have a default setting to load with all open.

What does the proposed API look like?

<Collapse defaultOpenAll={true} onChange={callback}>

[orianmarx]

Although you can set defaultActiveKey to an array of keys, it seems this can only be used to initialize a Collapse component. If you want to create say an expand all / collapse all button functionality, you can set a dynamic react key like this to get the component to rerender whenever you change the array of expanded keys: <Collapse defaultActiveKey={expandedKeys} key={expandedKeys.join()}>

[zhuangya]

key={expandedKeys.join()}

why do i have to write this key prop on <Collapse />? this is strange and nonintuitive