fix: hathor-core healthcheck

[luislhl]

Acceptance Criteria

• We should expect to receive a response with Content-Type text/html from hathor-core's health endpoint, and make aiohttp parse it without errors.
• After fix: healthcheck content-type hathor-core#1110 gets released, we will be able to revert this change. I preferred to do it instead of waiting for hathor-core, because it could take too long.

Security Checklist

• Make sure you do not include new dependencies in the project unless strictly necessary and do not include dev-dependencies as production ones. More dependencies increase the possibility of one of them being hijacked and affecting us.