1. Revert changes in setHeader from 5.2.9 that caused regression on self-copy. 2. Update copyHeaders/IfAbsent to ensure a copy of native headers. 3. Exit if source and target are the same instance, as an optimization. Closes gh-26155
1. Revert changes in setHeader from 5.2.9 that caused regression on self-copy. 2. Update copyHeaders/IfAbsent to ensure a copy of native headers. 3. Exit if source and target are the same instance, as an optimization. Closes gh-26155