Fix invalid JSON returned when marshaling dates

[hackerwins]

What this PR does / why we need it:

Fix invalid JSON returned when marshaling dates.

AS-IS

{
  "date": 2022-11-25T01:15:41+09:00
}

TO-BE

{
  "date": "2022-11-25T01:15:41+09:00"
}

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Additional documentation:

Checklist:

• Added relevant tests or not required
• Didn't break anything

[codecov]

Codecov Report

Merging #432 (5788641) into main (6e5869a) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #432   +/-   ##
=======================================
  Coverage   46.96%   46.96%           
=======================================
  Files          71       71           
  Lines        5913     5913           
=======================================
  Hits         2777     2777           
  Misses       2831     2831           
  Partials      305      305           

Impacted Files	Coverage Δ
pkg/document/crdt/primitive.go	96.18% <100.00%> (ø)

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