ChangeRequest: rc-collapse does not unmount children.

[stavenko]

There is a need do determine, if current PanelContent visible or active in it's children.
There are three methods to perform this:

1. Give PanelContent children isActive property.
2. Put active panel in modern react context.
3. Simply remove rendering of component and make user rely on componentWillMount, componentDidUnmount functions to track panels state.

Here is code snippet: render function of PanelContent:

 render() {
    this._isActived = this._isActived || this.props.isActive;
    if (!this._isActived) {
      return null;
    }
    const { prefixCls, isActive, children } = this.props;
    const contentCls = classnames({
      [`${prefixCls}-content`]: true,
      [`${prefixCls}-content-active`]: isActive,
      [`${prefixCls}-content-inactive`]: !isActive,
    });

    return (
      <div
        className={contentCls}
        role="tabpanel"
      >
        {isActive?<div className={`${prefixCls}-content-box`}>{children}</div>: null}

      </div>
    );
  },

In my humble opinion this change is trivial, does not breaks functionality or rc-collapse, and does not breaks user expectations about componentWillMount, componentDidUnmount behaviour.