Fix the Incorrectly generated attributes for toJsonString of Tree

[humdrum]

What this PR does / why we need it:

• Fix the Incorrectly generated attributes for toJsonString of Tree

The attributes of Tree node are contained as json String.
When initialize the JSONTreeNode, the json String converted to dictionary by toJSONObject in String+Extension.

yorkie-ios-sdk/Sources/Util/String+Extensions.swift

Lines 41 to 47 in ca1fbc0

	var toJSONObject: Any {
	if let data = self.data(using: .utf8) {
	return (try? JSONSerialization.jsonObject(with: data, options: [.fragmentsAllowed])) ?? self
	}
	return self
	}

The "jsonObject" that "JSONSerialization.jsonObject()" creates contains foundation types. (NSNumber, NSString...)
Since "jsonObject" isn't Encodable, they can't converted to string by old logic.
So I added JSONSerialization.data() to convert them to string.

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

All modified and coverable lines are covered by tests ✅

Comparison is base (ca1fbc0) 43.07% compared to head (d18643f) 43.31%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #131      +/-   ##
==========================================
+ Coverage   43.07%   43.31%   +0.23%     
==========================================
  Files         104      104              
  Lines       22958    22972      +14     
==========================================
+ Hits         9890     9951      +61     
+ Misses      13068    13021      -47     

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[myupage]

Works great