Improve table heading row conversion

src/tables.js

@@ -1,3 +1,5 @@
+var indexOf = Array.prototype.indexOf
+var every = Array.prototype.every
 var rules = {}
 
 rules.tableCell = {
@@ -13,10 +15,10 @@ rules.tableRow = {
     var borderCells = ''
     var alignMap = { left: ':--', right: '--:', center: ':-:' }
 
-    if (node.parentNode.nodeName === 'THEAD') {
+    if (isHeadingRow(node)) {
       for (var i = 0; i < node.childNodes.length; i++) {
-        var align = node.childNodes[i].attributes.align
         var border = '---'
+        var align = node.childNodes[i].attributes.align
 
         if (align) border = alignMap[align.value] || border
 
@@ -43,8 +45,38 @@ rules.tableSection = {
   }
 }
 
+// A tr is a heading row if:
+// - the parent is a THEAD
+// - or if its the first child of the TABLE or the first TBODY (possibly
+//   following a blank THEAD)
+// - and every cell is a TH
+function isHeadingRow (tr) {
+  var parentNode = tr.parentNode
+  return (
+    parentNode.nodeName === 'THEAD' ||
+    (
+      parentNode.firstChild === tr &&
+      (parentNode.nodeName === 'TABLE' || isFirstTbody(parentNode)) &&
+      every.call(tr.childNodes, function (n) { return n.nodeName === 'TH' })
+    )
+  )
+}
+
+function isFirstTbody (element) {
+  var previousSibling = element.previousSibling
+  return (
+    element.nodeName === 'TBODY' && (
+      !previousSibling ||
+      (
+        previousSibling.nodeName === 'THEAD' &&
+        /^\s*$/i.test(previousSibling.textContent)
+      )
+    )
+  )
+}
+
 function cell (content, node) {
-  var index = Array.prototype.indexOf.call(node.parentNode.childNodes, node)
+  var index = indexOf.call(node.parentNode.childNodes, node)
   var prefix = ' '
   if (index === 0) prefix = '| '
   return prefix + content + ' |'