fix(http): always set Content-Type to application/json

[rjan90]

JSON-RPC HTTP responses were returning Content-Type: text/plain; charset=utf-8 instead of the correct application/json. Added w.Header().Set("Content-Type", "application/json") in the ServeHTTP method for HTTP responses.

Ref: filecoin-project/lotus#13390

[Copilot]

Pull Request Overview

This PR fixes an issue where JSON-RPC HTTP responses were returning an incorrect Content-Type header (text/plain; charset=utf-8) instead of the proper application/json. The fix ensures JSON-RPC responses have the correct MIME type as expected by clients.

Key changes:

• Added Content-Type header setting in the ServeHTTP method
• Added comprehensive test coverage to verify the header fix and JSON response validity

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
server.go	Sets Content-Type header to "application/json" in ServeHTTP method
rpc_test.go	Adds test to verify Content-Type header is correctly set to "application/json"

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (master@bd8f54a). Learn more about missing BASE report.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #139   +/-   ##
=========================================
  Coverage          ?   72.30%           
=========================================
  Files             ?       13           
  Lines             ?     1997           
  Branches          ?        0           
=========================================
  Hits              ?     1444           
  Misses            ?      453           
  Partials          ?      100           

Files with missing lines	Coverage Δ
server.go	76.23% <100.00%> (ø)

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