Fix spring-doc issue #387

[VictorZeng]

What this PR does / why we need it?

Fix spring-doc issue #387 .

Summary of your change

Refactor FastJsonHttpMessageConverter writeInternal() method for JSON String.

Please indicate you've done the following:

• Made sure tests are passing and test coverage is added if needed.
• Made sure commit message follow the rule of Conventional Commits specification.
• Considered the docs impact and opened a new docs issue or PR with docs changes if needed.

[codecov-commenter]

Codecov Report

Merging #406 (7038d34) into main (e77e1ed) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##               main     #406      +/-   ##
============================================
- Coverage     69.77%   69.75%   -0.02%     
+ Complexity    11080    11077       -3     
============================================
  Files           516      515       -1     
  Lines         53275    53273       -2     
  Branches      11212    11213       +1     
============================================
- Hits          37172    37163       -9     
- Misses        11487    11493       +6     
- Partials       4616     4617       +1     

Impacted Files	Coverage Δ
...g/http/converter/FastJsonHttpMessageConverter.java	56.66% <100.00%> (+2.54%)	⬆️
core/src/main/java/com/alibaba/fastjson2/JSON.java	70.23% <0.00%> (-0.25%)	⬇️
...ain/java/com/alibaba/fastjson2/JSONWriterUTF8.java	76.82% <0.00%> (-0.25%)	⬇️
...in/java/com/alibaba/fastjson2/JSONWriterJSONB.java	58.14% <0.00%> (-0.19%)	⬇️
...rc/main/java/com/alibaba/fastjson2/JSONReader.java	71.63% <0.00%> (-0.09%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update e77e1ed...7038d34. Read the comment docs.