fix(render): improve JsonpJSON content type handling and simplify Context.JSONP

[1911860538]

This PR makes several improvements to JSONP handling:

Content Type Correction for JsonpJSON

• Modified JsonpJSON.Render to use 'application/json; charset=utf-8' when callback is empty
• This aligns with JSONP definition which requires a callback function wrapper
• Ensures proper content type matching for the actual data format being returned

Context.JSONP Method Simplification

• Refactored Context.JSONP to use a cleaner, one-line implementation
• Removed duplicate code and redundant condition checking

Test Updates

• Added TestRenderJsonpJSONWithEmptyCallback to verify correct content type

These changes improve code maintainability
while ensuring JSONP responses conform to proper standards.
According to JSONP (JSON with Padding) definition,
it is a technique for sending JSON data
without worrying about cross-domain issues
by wrapping the JSON response in a JavaScript function call.

Ref #1438

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.96%. Comparing base (3dc1cd6) to head (a14cf7c).
⚠️ Report is 275 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4427      +/-   ##
==========================================
- Coverage   99.21%   98.96%   -0.25%     
==========================================
  Files          42       44       +2     
  Lines        3182     2898     -284     
==========================================
- Hits         3157     2868     -289     
- Misses         17       19       +2     
- Partials        8       11       +3     

Flag	Coverage Δ
	?
--ldflags="-checklinkname=0" -tags sonic	98.95% <100.00%> (?)
-tags go_json	98.88% <100.00%> (?)
-tags nomsgpack	98.94% <100.00%> (?)
go-1.18	?
go-1.19	?
go-1.20	?
go-1.21	?
go-1.24	98.96% <100.00%> (?)
go-1.25	98.96% <100.00%> (?)
macos-latest	98.96% <100.00%> (-0.25%)	⬇️
ubuntu-latest	98.96% <100.00%> (-0.25%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[1911860538]

@appleboy CC