datetime: make date.new{} and date:set{} equivalent

Constructor date.new() and modifier date:set() should always produce same
result for all attributes combinations. Fixed the problem for
`timestamp` with `tzoffset`.

Fixes #6793

@TarantoolBot document
Title: datetime :set{} with tzoffset

Constructor `date.new()` and modifier `date:set()` should always produce
same result for all attributes combinations. Fixed the problem for
`timestamp` with `tzoffset`.

```
tarantool> date.new{tzoffset = '+0800', timestamp = 1630359071}
---
- 2021-08-30T21:31:11+0800
...

tarantool> date.new():set{tzoffset = '+0800', timestamp = 1630359071}
---
- 2021-08-30T21:31:11+0800
...
```