Use defaultNetwork in Script.toAddress

lib/script/script.js

@@ -702,11 +702,11 @@ Script.fromAddress = function(address) {
 };
 
 /**
+ * @param {Network} [network]
  * @return {Address} the associated address for this script
  */
 Script.prototype.toAddress = function(network) {
-  network = Networks.get(network);
-  $.checkArgument(network, 'Must provide a network');
+  network = Networks.get(network) || Networks.defaultNetwork;
   if (this.isPublicKeyHashOut() || this.isScriptHashOut()) {
     return new Address(this, network);
   }

test/script/script.js

@@ -592,6 +592,12 @@ describe('Script', function() {
   });
 
   describe('toAddress', function() {
+    it('uses default network', function() {
+      var stringAddress = '1NaTVwXDDUJaXDQajoa9MqHhz4uTxtgK14';
+      var address = new Address(stringAddress);
+      var script = new Script(address);
+      script.toAddress().toString().should.equal(stringAddress);
+    });
     it('for a P2PKH address', function() {
       var stringAddress = '1NaTVwXDDUJaXDQajoa9MqHhz4uTxtgK14';
       var address = new Address(stringAddress);