Punycode domain names only in known schemas

In urls like `skype:XXX` it prevents XXX to be parsed as domain
name and encoded with punycode.

lib/index.js

@@ -34,14 +34,24 @@ function validateLink(url) {
   return true;
 }
 
+var RECODE_HOSTNAME_FOR = [ 'http:', 'https:', 'mailto:' ];
+
 function normalizeLink(url) {
   var parsed = mdurl.parse(url, true);
 
   if (parsed.hostname) {
+    // Encode hostnames in urls like:
+    // `http://host/`, `https://host/`, `mailto:user@host`, `//host/`
+    //
+    // We don't encode unknown schemas, because it's likely that we encode
+    // something we shouldn't (e.g. `skype:name` treated as `skype:host`)
+    //
+    if (!parsed.protocol || RECODE_HOSTNAME_FOR.indexOf(parsed.protocol) >= 0) {
       try {
         parsed.hostname = punycode.toASCII(parsed.hostname);
       } catch(er) {}
     }
+  }
 
   return mdurl.encode(mdurl.format(parsed));
 }
@@ -50,10 +60,18 @@ function normalizeLinkText(url) {
   var parsed = mdurl.parse(url, true);
 
   if (parsed.hostname) {
+    // Encode hostnames in urls like:
+    // `http://host/`, `https://host/`, `mailto:user@host`, `//host/`
+    //
+    // We don't encode unknown schemas, because it's likely that we encode
+    // something we shouldn't (e.g. `skype:name` treated as `skype:host`)
+    //
+    if (!parsed.protocol || RECODE_HOSTNAME_FOR.indexOf(parsed.protocol) >= 0) {
       try {
         parsed.hostname = punycode.toUnicode(parsed.hostname);
       } catch(er) {}
     }
+  }
 
   return mdurl.decode(mdurl.format(parsed));
 }

test/fixtures/markdown-it/normalize.txt

@@ -43,6 +43,22 @@ Invalid punycode (non-ascii):
 <p><a href="http://xn--xn---emd.com/">http://xn–γ.com/</a></p>
 .
 
+Two slashes should start a domain:
+
+.
+[](//☃.net/)
+.
+<p><a href="//xn--n3h.net/"></a></p>
+.
+
+Don't encode domains in unknown schemas:
+
+.
+[](skype:γγγ)
+.
+<p><a href="skype:%CE%B3%CE%B3%CE%B3"></a></p>
+.
+
 Should auto-add protocol to autolinks:
 
 .