added test coverage for horizontal rulers and fixed rulers with underscores

tests/data/horizontal_rule.html

@@ -0,0 +1,28 @@
+<p>Horizontal line with dashes</p>
+<hr />
+<p>Horizontal line with asterisks</p>
+<hr />
+<p>Horizontal line with underscores</p>
+<hr />
+<p>Horizontal lines with more characters</p>
+<hr />
+<hr />
+<hr />
+<p>Horizontal lines with spaces</p>
+<hr />
+<hr />
+<hr />
+<hr />
+<hr />
+<hr />
+<hr />
+<hr />
+<hr />
+<p>Not horizontal lines</p>
+<ul>
+<li> _  -</li>
+</ul>
+<p>*--</p>
+<ul>
+<li>- - _ _ _</li>
+</ul>

tests/data/horizontal_rule.md

@@ -0,0 +1,47 @@
+Horizontal line with dashes
+
+---
+
+Horizontal line with asterisks
+
+***
+
+Horizontal line with underscores
+
+___
+
+Horizontal lines with more characters
+
+-------------
+
+*************
+
+_____________
+
+Horizontal lines with spaces
+
+- - - 
+
+-  - -
+
+- -  -
+
+--  -
+
+-  -  -
+
+* * *
+
+*  * *
+
+*  **
+
+__  __  ___ _  ___
+
+Not horizontal lines
+
+-  _  -
+
+*--
+
+- - - _ _ _