Fix initialization for [email protected]

[olzraiti]

This PR replaces the _onCloseButtonClick() call on initialization with close() according to [email protected] breaking changes. Leaflet's Popup API internals were reorganized according to their changelog:

• Reorganize DivOverlay/Popup/Tooltip APIs

[erictheise]

Thank you for your work, @olzraiti, gratifying to know people find this little library worth updating. Instead of this hard replace would you be willing to incorporate a Leaflet version check that'd maintain operability with versions earlier than 1.8?

[olzraiti]

Happy to help :) that should do the trick