ERP5Type.mixin.response_header_generator: Do not fail when removing a non-existent header.

Allows using an empty fallback value with "replace" set to trigger header
removal.
38 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
failed ERP5.CodingStyleTest-Master

02:22:21

passed ERP5.PerformanceTest-Master

00:24:44

failed ERP5.UnitTest-Master

04:15:11

failed ERP5.UnitTest-Master.Medusa

03:50:42

passed SlapOS.Eggs.UnitTest-Master.Python2

00:20:32

passed SlapOS.Eggs.UnitTest-Master.Python3

01:37:33

failed ERP5.CodingStyleTest-Master

03:25:01

failed ERP5.CodingStyleTest-Master

01:06:11

passed ERP5.PerformanceTest-Master

00:24:39

passed ERP5.PerformanceTest-Master

00:26:32

failed ERP5.UnitTest-Master

03:23:11

failed ERP5.UnitTest-Master

02:14:00

failed ERP5.UnitTest-Master

02:06:41

failed ERP5.UnitTest-Master

01:35:15

failed ERP5.UnitTest-Master

01:51:43

failed ERP5.UnitTest-Master.Medusa

03:01:22

failed ERP5.UnitTest-Master.Medusa

02:24:09

passed SlapOS.Eggs.UnitTest-Master.Python2

00:21:20

passed SlapOS.Eggs.UnitTest-Master.Python2

00:09:49

passed SlapOS.Eggs.UnitTest-Master.Python2

00:21:49

passed SlapOS.Eggs.UnitTest-Master.Python2

00:17:44

passed SlapOS.Eggs.UnitTest-Master.Python2

00:14:51

passed SlapOS.Eggs.UnitTest-Master.Python2

00:52:14

passed SlapOS.Eggs.UnitTest-Master.Python2

00:10:00

passed SlapOS.Eggs.UnitTest-Master.Python2

00:24:11

passed SlapOS.Eggs.UnitTest-Master.Python2

00:09:53

passed SlapOS.Eggs.UnitTest-Master.Python2

00:10:55

passed SlapOS.Eggs.UnitTest-Master.Python2

00:10:18

passed SlapOS.Eggs.UnitTest-Master.Python2

00:44:34

passed SlapOS.Eggs.UnitTest-Master.Python2

00:52:14

passed SlapOS.Eggs.UnitTest-Master.Python3

00:45:58

passed SlapOS.Eggs.UnitTest-Master.Python3

00:12:29

passed SlapOS.Eggs.UnitTest-Master.Python3

00:12:00

passed SlapOS.Eggs.UnitTest-Master.Python3

01:25:03

passed SlapOS.Eggs.UnitTest-Master.Python3

00:10:40

passed SlapOS.Eggs.UnitTest-Master.Python3

00:10:23

passed SlapOS.Eggs.UnitTest-Master.Python3

00:21:48

passed SlapOS.Eggs.UnitTest-Master.Python3

00:11:50