Encode Date object in return value of onCall functions

[takaaa220]

Description

refs: #988

Encode a Date object in return value of onCall functions to ISO formatted date string.

Code sample

onCall(() => { return { d: new Date() } })

The execution result of the above code changes as follows,

Before behavior:

{ "result": { "d": {} } }

After behavior:

{ "result": { "d": "2021-10-12T13:11:44.977Z" } }