Extra dash is generated when label is shorter than content (with padHeaderSeparator: true)

[gterras]

Hi and thanks for the great library,

I noticed

tablemark([{foo: null}, {foo: null}, {foo: null}], { padHeaderSeparator: true }) 

Will generate

| Foo   |
| :---- | // 4 dashes
| null  |
| null  |
| null  |

instead of

| Foo  |
| :--- | // 3 dashes
| null |
| null |
| null |

This triggers a reformat on VSC and Sublime.