setext underlines should not work on interrupted paragraphs

tests/data/setext.html

@@ -1,2 +0,0 @@
-<h2>line</h2>
-<h2>line</h2>

tests/data/setext.md

@@ -1,5 +0,0 @@
-line
--
-
-line
-----

tests/data/setext_header.html

@@ -0,0 +1,5 @@
+<h1>h1</h1>
+<h2>h2</h2>
+<h2>single character</h2>
+<p>not a header</p>
+<hr />

tests/data/setext_header.md

@@ -0,0 +1,12 @@
+h1
+==
+
+h2
+--
+
+single character
+-
+
+not a header
+
+------------