OnCall functions does not handle Date in return data

### [REQUIRED] Version info

**node: 14.17.2**

**firebase-functions: 3.15.7**

**firebase-tools: 9.20.0**

**firebase-admin: 9.12.0**

### [REQUIRED] Test case

```js
const functions = require("firebase-functions")

module.exports.testFunc = functions.https.onCall((data, req) => ({
d: new Date()
}))
```

### [REQUIRED] Steps to reproduce

1. Run emulator
2. Make functions request

### [REQUIRED] Expected behavior

Response must contain ISO formatted date. Like:
```json
{
"result": {
"d": "2021-10-12T13:11:44.977Z"
}
}
```

### [REQUIRED] Actual behavior

Response must contain ISO formatted date. Like:
```json
{
"result": {
"d": {}
}
}
```

Start with the functions.https.onCall entry point and reproduce the Date return value using the emulator and the versions listed in the report. Trace how the onCall response is serialized; done means the returned Date appears as an ISO-formatted string instead of an empty object.

Written by the indexing model from the issue text.