Table cell column spans are lost when rendering to CommonMark Markdown 1.2

When rendering a table to markdown/1.2, the colspan parameter of cells is ignored. Cells spanning several columns are rendered as a single cell, so the following cells of the row shift to the left and the table structure is lost (e.g. when converting a page to Markdown, or when saving a Markdown page edited with the WYSIWYG editor).

The Markdown 1.2 parser supports column spans written with consecutive pipes (e.g. | a || for a cell spanning 2 columns) and generates a colspan parameter for them.

Steps to reproduce

Parse and render back to markdown/1.2:

| a | b | c |
|---|---|---|
| d     || e |

Result

The second row is rendered with 2 normal cells (| d | e |), so e moves to the second column.

Expected

The column span is preserved (| d || e |).