caddy-frontend: Fix ATS issue with no-Content-Length caching

Fixes and tests https://github.com/apache/trafficserver/issues/7880
15 jobs for 1.0 in 0 seconds
Status Job ID Name Coverage
  External
passed SlapOS.Eggs.UnitTest-Master.Python2

00:32:43

passed SlapOS.Eggs.UnitTest-Master.Python3

00:19:22

passed SlapOS.SlapRunner.ResilienceTest-Master

00:23:45

failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

00:25:15

failed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster.Master

15:22:41

passed SlapOS.Eggs.UnitTest-Master.Python2

00:18:04

passed SlapOS.Eggs.UnitTest-Master.Python2

passed SlapOS.Eggs.UnitTest-Master.Python2

00:17:52

passed SlapOS.Eggs.UnitTest-Master.Python3

00:17:34

passed SlapOS.Eggs.UnitTest-Master.Python3

00:09:51

passed SlapOS.SlapRunner.ResilienceTest-Master

00:22:29

passed SlapOS.SlapRunner.ResilienceTest-Master

00:23:17

failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

00:27:37

failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

00:29:08

failed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster.Master

14:19:03