labels of reference links should be case insensitive

Parsedown.php

@@ -283,7 +283,7 @@ class Parsedown
 			
 			if ($pure_line[0] === '[' and preg_match('/^\[(.+?)\]:[ ]*([^ ]+)/', $pure_line, $matches))
 			{
-				$label = $matches[1];
+				$label = strtolower($matches[1]);
 				$url = trim($matches[2], '<>');
 				
 				$this->reference_map[$label] = $url;

tests/data/reference_link.html

@@ -2,6 +2,7 @@
 <p>Here's <a href="http://parsedown.org">one</a> on the next line.</p>
 <p>Here's <a href="http://parsedown.org/tests/">one</a> with a different URL.</p>
 <p>Here's <a href="http://parsedown.org">one</a> with a semantic name.</p>
+<p>Here's <a href="http://parsedown.org">one</a> with an upper case label definition.</p>
 <p>Here's <a href="http://parsedown.org">one</a> with definition name on the next line.</p>
 <p>Here's [one][404] with no definition.</p>
 <p>Here's an image: <img alt="Markdown Logo" src="/md.png"></p>

tests/data/reference_link.md

@@ -13,6 +13,10 @@ Here's [one][website] with a semantic name.
 
 [website]: http://parsedown.org
 
+Here's [one][case] with an upper case label definition.
+
+[CASE]: http://parsedown.org
+
 Here's [one]
 [website] with definition name on the next line.