RST writer: Avoid stack overflow with certain tables.

Closes #1197.

Note that there are still problems with the formatting of
the tables inside tables with output produced from the input
file in the original bug report.  But this fixes the stack
overflow problem.

src/Text/Pandoc/Writers/RST.hs

@@ -219,11 +219,15 @@ blockToRST (Table caption _ widths headers rows) =  do
                      else blankline <> text "Table: " <> caption'
   headers' <- mapM blockListToRST headers
   rawRows <- mapM (mapM blockListToRST) rows
-  let isSimple = all (==0) widths && all (all (\bs -> length bs <= 1)) rows
+  -- let isSimpleCell [Plain _] = True
+  --     isSimpleCell [Para _]  = True
+  --     isSimpleCell []        = True
+  --     isSimpleCell _         = False
+  -- let isSimple = all (==0) widths && all (all isSimpleCell) rows
   let numChars = maximum . map offset
   opts <- get >>= return . stOptions
   let widthsInChars =
-       if isSimple
+       if all (== 0) widths
           then map ((+2) . numChars) $ transpose (headers' : rawRows)
           else map (floor . (fromIntegral (writerColumns opts) *)) widths
   let hpipeBlocks blocks = hcat [beg, middle, end]