RST writers: Use grid tables for 1-column tables.

With simple tables, we have a clash with heading syntax.
Closes #5936.

src/Text/Pandoc/Writers/RST.hs

@@ -294,7 +294,7 @@ blockToRST (Table caption aligns widths headers rows) = do
          modify $ \st -> st{ stOptions = oldOpts }
          return result
   opts <- gets stOptions
-  let isSimple = all (== 0) widths
+  let isSimple = all (== 0) widths && length widths > 1
   tbl <- if isSimple
             then do
               tbl' <- simpleTable opts blocksToDoc headers rows

test/command/5936.md

@@ -0,0 +1,40 @@
+```
+% pandoc -f gfm -t rst
+| Name |
+| ---- |
+| Foo  |
+| Bar  |
+| Baz  |
+| Quux |
+^D
++------+
+| Name |
++======+
+| Foo  |
++------+
+| Bar  |
++------+
+| Baz  |
++------+
+| Quux |
++------+
+```
+
+```
+% pandoc -f gfm -t rst
+| Name | Number |
+| ---- | ------ |
+| Foo  | 3 |
+| Bar  | 4 |
+| Baz  | 2 |
+| Quux | 1 |
+^D
+==== ======
+Name Number
+==== ======
+Foo  3
+Bar  4
+Baz  2
+Quux 1
+==== ======
+```