Fix sole blank line in fenced code

Closes GH-24.

Author: wooorm

lib/util/format-code-as-indented.js

@@ -9,6 +9,6 @@ function formatCodeAsIndented(node, context) {
     // And there’s a non-whitespace character…
     /[^ \r\n]/.test(node.value) &&
     // And the value doesn’t start or end in a blank…
-    !/^[\t ]*[\r\n]|[\r\n][\t ]*$/.test(node.value)
+    !/^[\t ]*(?:[\r\n]|$)|(?:^|[\r\n])[\t ]*$/.test(node.value)
   )
 }

test.js

@@ -2882,5 +2882,13 @@ test('roundtrip', function (t) {
     'should not collapse escapes (2)'
   )
 
+  doc = '```\n	\n```\n'
+
+  t.equal(
+    to(from(doc)),
+    doc,
+    'should roundtrip a sole blank line in fenced code'
+  )
+
   t.end()
 })