fix: prevent superfluous response.WriteHeader call in EncodeJSONResponse

[Kubudak90]

Fixes #407

Problem

EncodeJSONResponse was calling w.WriteHeader(status) before attempting to encode the JSON. If the JSON encoding failed, http.Error() would try to write the header again, causing the error:

http: superfluous response.WriteHeader call from github.com/coinbase/rosetta-sdk-go/server.EncodeJSONResponse

This occurred during high-concurrency operations when JSON encoding occasionally failed.

Solution

Encode JSON to a bytes.Buffer first to validate it before writing any headers. Only write the status header after successful encoding. This ensures headers are written exactly once.

Changes

• Encode JSON to bytes.Buffer first to catch encoding errors
• Only write status header after successful encoding
• Return JSON error response instead of http.Error on encoding failure (maintains JSON content type)
• Rename router variable to muxRouter to avoid shadowing the import

Testing

The fix prevents the race condition where headers could be written twice. The buffer approach is a common pattern in Go HTTP handlers to ensure atomic header writes.

[cb-heimdall]

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