Table cell alignment is lost when rendering to CommonMark Markdown 1.2

When rendering a table to markdown/1.2, the cell alignment (align parameter, e.g. coming from a Markdown table with :---: separators) is not rendered: the separator row is always | — |.

Steps to reproduce

Parse and render back to markdown/1.2:

| a | b |
|:--|--:|
| c | d |

Result

The alignment markers are lost (| — | — |).

Expected

The column alignment is preserved (:--, -:, :--:).