Honor publicKey.network on create multisig address

Closes #870

lib/address.js

@@ -262,6 +262,7 @@ Address._transformScript = function(script, network){
  */
 Address.createMultisig = function(publicKeys, threshold, network) {
   var Script = require('./script');
+  network = network || publicKeys[0].network;
   return new Address(Script.buildMultisigOut(publicKeys, threshold), network || Networks.defaultNetwork);
 };