The default tooltip/popup pane isn't used when adding an element to a custom pane.

[RichardWithnell]

• All peer dependencies are installed: React, ReactDOM and Leaflet.
• Using a supported version of React and ReactDOM (v15.x or v16.x).
• Using the supported version of Leaflet (v1.2.x) and its corresponding CSS file is loaded.
• Using the latest stable version of React-Leaflet.
• The issue has not already been reported.
• Make sure you have followed the quick start guide for Leaflet.
• Make sure you have fully read the documentation and that you understand the technical considerations.

Expected behavior

When adding an element to a custom pane, any associated popup/tooltip should still be added to the default pane for that element, unless it is explicitly overwritten using the pane prop.

This appears to be the default behaviour in Leaflet.

Actual behavior

The popup is added to the same custom pane as the leaflet element. The popup won't display correctly, appearing behind the elements already in the pane.

Steps to reproduce

Clicking the yellow rectangle in the top map of the following bin:

https://www.webpackbin.com/bins/-Kw1QYT2vnFMlRLt8xGe

The bottom map recreates the same example with pure Leaflet, demonstrating the popup being added to the default instead of the custom panel.

[PaulLeCam]

Should be fixed in v1.7.1 thanks to your PR, please try it out.