Require space before title in reference link.

See commonmark/cmark#263.

Author: jgm

lib/inlines.js

@@ -808,7 +808,9 @@ var parseReference = function(s, refmap) {
 
     var beforetitle = this.pos;
     this.spnl();
-    title = this.parseLinkTitle();
+    if (this.pos !== beforetitle) {
+        title = this.parseLinkTitle();
+    }
     if (title === null) {
         title = '';
         // rewind before spaces