NotificationHub API cannot set the `registrationTtl` property anymore

After tested, seems NotificationHub API cannot set the `registrationTtl` property anymore.

PUT https://management.azure.com/subscriptions/xx-xx-xx-xx/resourceGroups/acctestRGpol-test012/providers/Microsoft.NotificationHubs/namespaces/acctestnhn-test012/notificationHubs/acctestnh-test013?api-version=2017-04-01

Request body:
```
{
"location": "westeurope",
"sku": {
"name": "Free"
},
"properties": {
"registrationTtl": "PT1H"
}
}
```

Response:
```
{
"properties": {
"registrationTtl": "10675199.02:48:05.4775807",
"authorizationRules": [],
"dailyMaxActiveDevices": 0
},
"id": "/subscriptions/xx-xx-xx-xx/resourceGroups/acctestRGpol-test012/providers/Microsoft.NotificationHubs/namespaces/acctestnhn-test012/NotificationHubs/acctestnh-test013",
"name": "acctestnh-test013",
"type": "Microsoft.NotificationHubs/namespaces/notificationHubs",
"location": "West Europe"
}
```

Start by locating the Microsoft.NotificationHubs notificationHubs specification for API version 2017-04-01 and compare its handling of the registrationTtl property with the PUT request and response shown here. Confirm the intended duration format and identify a specification-level regression test or example that demonstrates registrationTtl can be set successfully.

Written by the indexing model from the issue text.