Append headers instead of replacing them when headers is not nil
Append headers instead of replacing them when headers is not nil

Signed-off-by: 81390d236d295292853435c04577dfc593a96428@lalitadithya.com <81390d236d295292853435c04577dfc593a96428@lalitadithya.com>

jetstack-experimental/cert-manager,jetstack/cert-manager,jetstack-experimental/cert-manager,cert-manager/cert-manager,cert-manager/cert-manager,jetstack/cert-manager,cert-manager/cert-manager,jetstack-experimental/cert-manager,jetstack/cert-manager