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]

Beginne beim WebApp update().defineSslBinding()-Ablauf und verfolge AppServiceCertificateImpl bis zu CertificatesInner.createOrUpdateAsync, wo der Stacktrace die Validierung eines Null-Passworts meldet. Reproduziere das bestehende AppServiceCertificateOrder-Szenario und untersuche den generierten Request-Pfad. Fertig ist die Änderung, wenn ein bestehender Zertifikatsauftrag ohne einen unnötigen Passwort-Validierungsfehler angehängt werden kann.

Vom Indexierungsmodell aus dem Issue-Text verfasst.