RST writer generate invalid output for empty table rows

[pyhedgehog]

Scenario

Let's start with some correct RST:

+---+---+
| 1 | 2 |
+---+---+
|   |   |
+---+---+

Then we can do:

# pandoc -r rst -w html -o pandoc_bug.html pandoc_bug.rst
# pandoc -r html -w rst -o pandoc_bug-o.rst pandoc_bug.html

html output is correct:

<table style="width:11%;">
<colgroup>
<col style="width: 5%" />
<col style="width: 5%" />
</colgroup>
<tbody>
<tr class="odd">
<td>1</td>
<td>2</td>
</tr>
<tr class="even">
<td></td>
<td></td>
</tr>
</tbody>
</table>

But back to RST creates invalid file:

+--------------------------------------+--------------------------------------+
| 1                                    | 2                                    |
+--------------------------------------+--------------------------------------+
+--------------------------------------+--------------------------------------+

Same will happens if we use some other intermediate format supporting tables:

# pandoc -r rst -w odt -o pandoc_bug.odt pandoc_bug.rst
# pandoc -r odt -w rst -o pandoc_bug-o.rst pandoc_bug.odt

Environment

OS: Windows 10 x64
PanDoc: 1.19.2.1

C:\>clist -lo pandoc
Chocolatey v0.10.3
pandoc 1.19.2.1
1 packages installed.
C:\>pandoc -v
pandoc.EXE 2.0
Compiled with pandoc-types 1.17.0.4, texmath 0.9, skylighting 0.1.1.4
Default user data directory: C:\Users\username\AppData\Roaming\pandoc
Copyright (C) 2006-2016 John MacFarlane
Web:  http://pandoc.org
This is free software; see the source for copying conditions.
There is no warranty, not even for merchantability or fitness
for a particular purpose.

[jgm]

The markdown writer/reader gets this right:

% pandoc -f markdown -t markdown
+---+---+
| 1 | 2 |
+---+---+
|   |   |
+---+---+
^D
+---+---+
| 1 | 2 |
+---+---+
|   |   |
+---+---+

but I can confirm that (even in the dev version) the rst reader does not. Simple test case:

% pandoc -t rst -f rst
+---+---+
| 1 | 2 |
+---+---+
|   |   |
+---+---+
^D
+------+------+
| 1    | 2    |
+------+------+
+------+------+

Note that there are two problems: the missing "sides" of the bottom table row and the widening of the cells.

Note that the markdown and rst readers both parse this as

[Table [] [AlignDefault,AlignDefault] [5.555555555555555e-2,5.555555555555555e-2]
 [[]
 ,[]]
 [[[Plain [Str "1"]]
  ,[Plain [Str "2"]]]
 ,[[]
  ,[]]]]

so the problems concern the RST writer.