added extension test that verifies that footnote labels get href escaped.

Author: phillmv

test/extensions.txt

@@ -722,6 +722,23 @@ This footnote is referenced[^a-footnote] multiple times, in lots of different pl
 </section>
 ````````````````````````````````
 
+## Footnote reference labels are href escaped
+
+```````````````````````````````` example
+Hello[^"><script>alert(1)</script>]
+
+[^"><script>alert(1)</script>]: pwned
+.
+<p>Hello<sup class="footnote-ref"><a href="#fn:%22%3E%3Cscript%3Ealert(1)%3C/script%3E" id="fnref:%22%3E%3Cscript%3Ealert(1)%3C/script%3E">1</a></sup></p>
+<section class="footnotes">
+<ol>
+<li id="fn:%22%3E%3Cscript%3Ealert(1)%3C/script%3E">
+<p>pwned <a href="#fnref:%22%3E%3Cscript%3Ealert(1)%3C/script%3E" class="footnote-backref">↩</a></p>
+</li>
+</ol>
+</section>
+````````````````````````````````
+
 ## Interop
 
 Autolink and strikethrough.