Handle Tree.toXML to return proper XML string

[raararaara]

What this PR does / why we need it?

When attempting to directly convert the object resulting from JSON.parse() into a string during the toXML() process for RHT, issues may arise. The conversion of the object to a string may result in an output such as [object Object], which needs to be addressed. This PR handles the conversion of objects to strings to prevent unexpected results.

Any background context you want to provide?

In the toXML() process for RHT, the value obtained from JSON.parse() was directly converted to a string. This conversion led to undesired outcomes when the object contained in the value was transformed into the string [object Object]. To mitigate this issue, adjustments were made to properly handle the conversion of objects to strings.

What are the relevant tickets?

Fixes #

Checklist

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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.74%. Comparing base (f765d1c) to head (0c4c764).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #805      +/-   ##
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+ Coverage   80.67%   80.74%   +0.06%     
==========================================
  Files          59       59              
  Lines        4476     4481       +5     
  Branches      908      908              
==========================================
+ Hits         3611     3618       +7     
  Misses        601      601              
+ Partials      264      262       -2     

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[raararaara]

In 'concurrently-edit-style-test', the result of first test case was as follows, so I handled an exception for the case where it is not a nested object, that is, 'string'. I am wondering if there is a better way.

yorkie-js-sdk/test/integration/tree_concurrency_test.ts

Line 603 in 9676575

const initialXML = `<r><p color="red">a</p><p color="red">b</p><p color="red">c</p></r>`;

Expected :<r><p color="red">a</p><p color="red">b</p><p color="red">c</p></r>
Actual   :<r><p color="\"red\"">a</p><p color="\"red\"">b</p><p color="\"red\"">c</p></r>

[hackerwins]

Thanks for your contribution.