Fix quill example page image rendering issue

[JOOHOJANG]

What this PR does / why we need it?

reported by @chacha912 (#491)

The cause of this issue is that JSON.stringify method is invoked twice.

• 

• yorkie-js-sdk/src/document/crdt/text.ts

  Line 472 in 5a0d2b4

  public stringifyAttributes(attributes: A): Record<string, string> {

• 

• yorkie-js-sdk/public/quill.html

  Line 173 in 5a0d2b4

  embed: JSON.stringify(op.insert),

Object {image: "data:..."} is over-stringified ('"{"image": "data:...."}"')
Therefore, I removed the previously called part because it calls JSON.stringify anyway.

Any background context you want to provide?

What are the relevant tickets?

Fixes #491

Checklist

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

[codecov]

Codecov Report

Merging #504 (f0111de) into main (3b313a5) will increase coverage by 0.47%.
The diff coverage is 87.75%.

@@            Coverage Diff             @@
##             main     #504      +/-   ##
==========================================
+ Coverage   89.69%   90.17%   +0.47%     
==========================================
  Files          69       70       +1     
  Lines        5669     6145     +476     
  Branches      549      576      +27     
==========================================
+ Hits         5085     5541     +456     
- Misses        390      392       +2     
- Partials      194      212      +18     

Impacted Files	Coverage Δ
src/client/attachment.ts	77.77% <0.00%> (+1.03%)	⬆️
src/document/crdt/array.ts	81.81% <0.00%> (-1.91%)	⬇️
src/document/json/array.ts	86.43% <0.00%> (-1.33%)	⬇️
src/document/operation/operation.ts	100.00% <ø> (ø)
test/helper/helper.ts	82.22% <ø> (+8.14%)	⬆️
test/integration/object_test.ts	100.00% <ø> (ø)
src/client/client.ts	73.53% <25.00%> (-0.97%)	⬇️
src/document/operation/select_operation.ts	50.00% <42.85%> (+11.11%)	⬆️
src/document/operation/style_operation.ts	54.16% <50.00%> (+4.16%)	⬆️
src/document/operation/increase_operation.ts	66.66% <57.14%> (+2.38%)	⬆️
... and 13 more

... and 5 files with indirect coverage changes

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

[hackerwins]

Thanks for your contribution. 👍