Tables should always have at least one heading row

[jackycute]

Hi @domchristie
I've been using this project for a long time, it's awesome!
Thank you for creating this and please keep work on.
Really looking forward to this project 😄

Back to this issue.
GFM tables need at least one heading row syntax to make a table.
But current parser would only parse the heading row in some situation. (https://github.com/domchristie/turndown-plugin-gfm/blob/master/src/tables.js#L55)
So I would propose to you that make the export table markdown always have at least one heading row even the original HTML doesn't meet the rules.

For example:

<table>
  <tr>
    <td>1</td>
    <td>2</td>
  </tr>
  <tr>
    <td>3</td>
    <td>4</td>
  </tr>
</table>

Will result:

| 1 | 2 |
|---|---|
| 3 | 4 |

Thank you very much!