Router "xyz" is invalid: spec.tls.certificate: Invalid value: "...": field is immutable in Red Hat OpenShift Container Platform 3.6

Issue

• We have users who can't edit TLS certificate of a route in one of their projects. The error message they are seeing is Reason: Router "<route-name>" is invalid: spec.tls.certificate: Invalid value: "<content-of-the-tls-certificate>": field is immutable'.. Any idea what is wrong here and how to fix this problem?