[FIX] Resizing table with merged cells should work as expected.

[hedgerwang]

About

After resizing a cell that spans multiple columns, the table's layout will appear broken (e.g. wider or narrower than expected).

Test Plan

• Test the following snippet https://gist.github.com/hedgerwang/f453d8370b004047cc33bf91929b596f
• Test with url http://localhost:3001/convert.html

Before	After
Resizing a simple table works fine, but resizing a table with merged cells will end up with wrong layout	Works as expected

[hedgerwang]

This also addresses the issue #73