Samuel has the correct answer in terms of doing it from a server perspective on the API Gateway. The header comes from Apache Tomcat which is used by the API Gateway for various types of connections.
As an FYI, we actually have a KB article on this very topic already laid out with instructions, located here for your reference. The caveat is you must be signed in to the Layer 7 Support Portal for that to be readable.
Sincerely,
Dustin Dauncey
Support Engineer, Global Customer Success
Email: API-Support@ca.com
Phone: +1 800 225 5224
Outside of North America - ca.com/us/worldwide.aspx
CA API Management Community: ca.com/talkapi