bip21: check for arg `r=` and set merchant.

lib/BIP21.js

@@ -62,6 +62,7 @@ BIP21.prototype.parse = function(uri) {
   for (var arg in info.query) {
     var val = info.query[arg];
     if (arg === 'amount') val = Number(val);
+    if (arg === 'r') this.data.merchant = val;
     this.data[arg] = val;
   }
 }