getUser() return value contains passwordHash and passwordSalt

The `UserRecord` returned by `admin.auth().getUser()` contains `passwordHash` and `passwordSalt` fields.

```
const admin = require('firebase-admin')
admin.initializeApp();
admin.auth().getUser(uid)
.then((user) => console.log(user));
```

This resulted in:

```
UserRecord {
uid: 'e1b2NmnasZXw0QtpYFcZ88IeK5t1',
email: '********@gmail.com',
emailVerified: true,
displayName: undefined,
photoURL: undefined,
phoneNumber: undefined,
disabled: false,
metadata:
UserMetadata {
creationTime: 'Wed, 03 Apr 2019 23:36:27 GMT',
lastSignInTime: 'Thu, 04 Apr 2019 00:38:10 GMT' },
providerData:
[ UserInfo {
uid: '********@gmail.com',
displayName: undefined,
email: '********@gmail.com',
photoURL: undefined,
providerId: 'password',
phoneNumber: undefined } ],
passwordHash: 'UkVEQUNURUQ=',
passwordSalt: undefined,
customClaims: undefined,
tokensValidAfterTime: 'Wed, 03 Apr 2019 23:36:27 GMT' }
```

This is contrary to the [documented behavior](https://firebase.google.com/docs/reference/admin/node/admin.auth.UserRecord#passwordHash):

> passwordHash
> (string or undefined)
>
> The user’s hashed password (base64-encoded), only if Firebase Auth hashing algorithm (SCRYPT) is used. If a different hashing algorithm had been used when uploading this user, as is typical when migrating from another Auth system, this will be an empty string. If no password is set, this is null. This is only available when the user is obtained from listUsers().