WebApp update failing with password validation when using existing AppServiceCertificateOrder

We have the following code that tries to update an existing WebApp with an existing AppServiceCertificateOrder.

```
AppServiceCertificateOrder certificateOrder = azure.appServices().certificateOrders().getById(OUR_APPSERVICECERT_ID);
webApp.update().defineSslBinding()
.forHostname(OUR_HOSTNAME)
.withExistingAppServiceCertificateOrder(certificateOrder)
.withSniBasedSsl()
.attach()
.apply();
```

The certificateOrder that is retrieved seems correct when debugging, so our ID is not the problem. However, when the update is applied, the API fails with the following error (the request is never sent, if I debugged correctly). I don't see where or why I would need to provide a password to use an existing AppServiceCertificate, though, so this appears to be a bug or I am using the API incorrectly.

> java.lang.IllegalArgumentException: password is required and cannot be null.
at com.microsoft.rest.Validator.validate(Validator.java:85) ~[client-runtime-1.3.0.jar:?]
at com.microsoft.azure.management.appservice.implementation.CertificatesInner.createOrUpdateWithServiceResponseAsync(CertificatesInner.java:491) ~[azure-mgmt-appservice-1.6.0.jar:1.6.0]
at com.microsoft.azure.management.appservice.implementation.CertificatesInner.createOrUpdateAsync(CertificatesInner.java:460) ~[azure-mgmt-appservice-1.6.0.jar:1.6.0]
at com.microsoft.azure.management.appservice.implementation.AppServiceCertificateImpl$3.call(AppServiceCertificateImpl.java:157) ~[azure-mgmt-appservice-1.6.0.jar:1.6.0]
at com.microsoft.azure.management.appservice.implementation.AppServiceCertificateImpl$3.call(AppServiceCertificateImpl.java:154) ~[azure-mgmt-appservice-1.6.0.jar:1.6.0]
at rx.internal.operators.OnSubscribeMap$MapSubscriber.onNext(OnSubscribeMap.java:69) ~[rxjava-1.2.4.jar:1.2.4]
at rx.internal.producers.SingleProducer.request(SingleProducer.java:65) ~[rxjava-1.2.4.jar:1.2.4]
at rx.Subscriber.setProducer(Subscriber.java:211) ~[rxjava-1.2.4.jar:1.2.4]
at rx.internal.operators.OnSubscribeMap$MapSubscriber.setProducer(OnSubscribeMap.java:102) ~[rxjava-1.2.4.jar:1.2.4]
at rx.internal.operators.OperatorSingle$ParentSubscriber.onCompleted(OperatorSingle.java:110) ~[rxjava-1.2.4.jar:1.2.4]
at rx.internal.operators.DeferredScalarSubscriber.complete(DeferredScalarSubscriber.java:102) ~[rxjava-1.2.4.jar:1.2.4]
at rx.internal.operators.OnSubscribeTakeLastOne$TakeLastOneSubscriber.onCompleted(OnSubscribeTakeLastOne.java:57) ~[rxjava-1.2.4.jar:1.2.4]
at rx.observers.SerializedObserver.onCompleted(SerializedObserver.java:176) ~[rxjava-1.2.4.jar:1.2.4]
at rx.observers.SerializedSubscriber.onCompleted(SerializedSubscriber.java:64) ~[rxjava-1.2.4.jar:1.2.4]
at rx.internal.operators.OnSubscribeConcatMap$ConcatMapSubscriber.drain(OnSubscribeConcatMap.java:246) ~[rxjava-1.2.4.jar:1.2.4]
at rx.internal.operators.OnSubscribeConcatMap$ConcatMapSubscriber.onCompleted(OnSubscribeConcatMap.java:169) ~[rxjava-1.2.4.jar:1.2.4]
at rx.internal.operators.OnSubscribeFromArray$FromArrayProducer.slowPath(OnSubscribeFromArray.java:106) ~[rxjava-1.2.4.jar:1.2.4]
at rx.internal.operators.OnSubscribeFromArray$FromArrayProducer.request(OnSubscribeFromArray.java:63) ~[rxjava-1.2.4.jar:1.2.4]
at rx.Subscriber.setProducer(Subscriber.java:211) ~[rxjava-1.2.4.jar:1.2.4]
at rx.internal.operators.OnSubscribeFromArray.call(OnSubscribeFromArray.java:32) ~[rxjava-1.2.4.jar:1.2.4]
at rx.internal.operators.OnSubscribeFromArray.call(OnSubscribeFromArray.java:24) ~[rxjava-1.2.4.jar:1.2.4]
at rx.Observable.unsafeSubscribe(Observable.java:10142) ~[rxjava-1.2.4.jar:1.2.4]
at rx.internal.operators.OnSubscribeConcatMap.call(OnSubscribeConcatMap.java:94) ~[rxjava-1.2.4.jar:1.2.4]
at rx.internal.operators.OnSubscribeConcatMap.call(OnSubscribeConcatMap.java:42) ~[rxjava-1.2.4.jar:1.2.4]
at rx.Observable.unsafeSubscribe(Observable.java:10142) ~[rxjava-1.2.4.jar:1.2.4]
at rx.internal.operators.DeferredScalarSubscriber.subscribeTo(DeferredScalarSubscriber.java:153) ~[rxjava-1.2.4.jar:1.2.4]
at rx.internal.operators.OnSubscribeTakeLastOne.call(OnSubscribeTakeLastOne.java:32) ~[rxjava-1.2.4.jar:1.2.4]
at rx.internal.operators.OnSubscribeTakeLastOne.call(OnSubscribeTakeLastOne.java:22) ~[rxjava-1.2.4.jar:1.2.4]
at rx.internal.operators.OnSubscribeLift.call(OnSubscribeLift.java:48) ~[rxjava-1.2.4.jar:1.2.4]
at rx.internal.operators.OnSubscribeLift.call(OnSubscribeLift.java:30) ~[rxjava-1.2.4.jar:1.2.4]
at rx.Observable.unsafeSubscribe(Observable.java:10142) ~[rxjava-1.2.4.jar:1.2.4]
at rx.internal.operators.OnSubscribeMap.call(OnSubscribeMap.java:48) ~[rxjava-1.2.4.jar:1.2.4]
at rx.internal.operators.OnSubscribeMap.call(OnSubscribeMap.java:33) ~[rxjava-1.2.4.jar:1.2.4]
at rx.internal.operators.OnSubscribeLift.call(OnSubscribeLift.java:48) ~[rxjava-1.2.4.jar:1.2.4]
at rx.internal.operators.OnSubscribeLift.call(OnSubscribeLift.java:30) ~[rxjava-1.2.4.jar:1.2.4]
at rx.Observable.unsafeSubscribe(Observable.java:10142) ~[rxjava-1.2.4.jar:1.2.4]
at rx.internal.operators.OnSubscribeMap.call(OnSubscribeMap.java:48) ~[rxjava-1.2.4.jar:1.2.4]
at rx.internal.operators.OnSubscribeMap.call(OnSubscribeMap.java:33) ~[rxjava-1.2.4.jar:1.2.4]
at rx.Observable.unsafeSubscribe(Observable.java:10142) ~[rxjava-1.2.4.jar:1.2.4]
at rx.internal.operators.OperatorSubscribeOn$1.call(OperatorSubscribeOn.java:94) ~[rxjava-1.2.4.jar:1.2.4]
at rx.internal.schedulers.CachedThreadScheduler$EventLoopWorker$1.call(CachedThreadScheduler.java:230) ~[rxjava-1.2.4.jar:1.2.4]
at rx.internal.schedulers.ScheduledAction.run(ScheduledAction.java:55) ~[rxjava-1.2.4.jar:1.2.4]
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511) ~[?:1.8.0_51]
at java.util.concurrent.FutureTask.run(FutureTask.java:266) ~[?:1.8.0_51]
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$201(ScheduledThreadPoolExecutor.java:180) ~[?:1.8.0_51]
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:293) ~[?:1.8.0_51]
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142) ~[?:1.8.0_51]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617) ~[?:1.8.0_51]
at java.lang.Thread.run(Thread.java:745) [?:1.8.0_51]

Start at the WebApp update().defineSslBinding() flow and follow AppServiceCertificateImpl into CertificatesInner.createOrUpdateAsync, where the stack trace reports the null-password validation. Reproduce the existing AppServiceCertificateOrder scenario and inspect the generated request path. Done means an existing certificate order can be attached without an unnecessary password validation failure.

索引モデルが issue の本文から書いたものです。