Double URL escape

[scossu]

Percent-escaped characters in URL are double-escaped when converted to a tags.

To reproduce (lua 5.4):

md = require "discount"
mdstr = "[Cenomanian Turonian Boundary Event](https://en.wikipedia.org/wiki/Cenomanian%E2%80%93Turonian_boundary_event)"
md(mdstr)
--> <p><a href="https://en.wikipedia.org/wiki/Cenomanian%25E2%2580%2593Turonian_boundary_event">Cenomanian Turonian Boundary Event</a></p>

Note %E2%80%93 becoming %25E2%2580%2593 (%25 is % url-encoded).