[e.cash] Header implementation bug fix

Summary: Fix syntax in implemented headers

Test Plan:

docker build -t ecash_local .
docker run -it --rm -p 3000:3000 ecash_local
curl -I http://localhost:3000

Confirm terminal output includes headers

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15227