Make attributes display properly in dashboard

[YoonKiJin]

What this PR does / why we need it:

(Bug fix)

Which issue(s) this PR fixes:

Fixes yorkie-team/dashboard#128

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

[CLAassistant]

All committers have signed the CLA.

[hackerwins]

Oh, you sent your first PR. 💪

Next, it would be good to do the below two things:

1. Fix the error on the CI test
2. Add related test code

[hackerwins]

Thank you for reflecting on the requests.

Next, we need to remove unnecessary changes from this PR.
crdt/internal_document.go, database/database.go, memory/database.go

[codecov]

Codecov Report

Merging #564 (a2998fa) into main (abf17ac) will decrease coverage by 0.04%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main     #564      +/-   ##
==========================================
- Coverage   51.94%   51.91%   -0.04%     
==========================================
  Files          67       67              
  Lines        6847     6851       +4     
==========================================
  Hits         3557     3557              
- Misses       2851     2855       +4     
  Partials      439      439              

Impacted Files	Coverage Δ
pkg/document/crdt/tree.go	64.39% <0.00%> (-1.00%)	⬇️

[hackerwins]

LGTM. 👍