kubernetes: [e2e test failure] Simple pod should support exec through kubectl proxy

/cc @kubernetes/sig-cli-bugs

This test has started failing on GKE and GCI-GKI e2e tests:

https://k8s-testgrid.appspot.com/release-master-blocking#gci-gke https://k8s-testgrid.appspot.com/release-master-blocking#gke

Example failure

https://k8s-gubernator.appspot.com/build/kubernetes-jenkins/logs/ci-kubernetes-e2e-gke/12920

/go/src/k8s.io/kubernetes/_output/dockerized/go/src/k8s.io/kubernetes/test/e2e/kubectl/kubectl.go:457
Expected error:
    <exec.CodeExitError>: {
        Err: {
            s: "error running &{/workspace/kubernetes/platforms/linux/amd64/kubectl [kubectl --server=https://35.188.213.188 --kubeconfig=/tmp/gke-kubecfg386178168 --server=http://127.0.0.1:45529 --namespace=e2e-tests-kubectl-nlzj2 exec nginx echo running in container] []  <nil>  W0810 18:25:44.244920    4112 http.go:342] Error reading backend response: unexpected EOF\nerror: error sending request: Post http://127.0.0.1:45529/api/v1/namespaces/e2e-tests-kubectl-nlzj2/pods/nginx/exec?command=echo&command=running&command=in&command=container&container=nginx&container=nginx&stderr=true&stdout=true: unexpected EOF\n [] <nil> 0xc4211d3830 exit status 1 <nil> <nil> true [0xc4203e6828 0xc4203e6840 0xc4203e6858] [0xc4203e6828 0xc4203e6840 0xc4203e6858] [0xc4203e6838 0xc4203e6850] [0x11bf750 0x11bf750] 0xc420f2db60 <nil>}:\nCommand stdout:\n\nstderr:\nW0810 18:25:44.244920    4112 http.go:342] Error reading backend response: unexpected EOF\nerror: error sending request: Post http://127.0.0.1:45529/api/v1/namespaces/e2e-tests-kubectl-nlzj2/pods/nginx/exec?command=echo&command=running&command=in&command=container&container=nginx&container=nginx&stderr=true&stdout=true: unexpected EOF\n\nerror:\nexit status 1\n",
        },
        Code: 1,
    }
    error running &{/workspace/kubernetes/platforms/linux/amd64/kubectl [kubectl --server=https://35.188.213.188 --kubeconfig=/tmp/gke-kubecfg386178168 --server=http://127.0.0.1:45529 --namespace=e2e-tests-kubectl-nlzj2 exec nginx echo running in container] []  <nil>  W0810 18:25:44.244920    4112 http.go:342] Error reading backend response: unexpected EOF
    error: error sending request: Post http://127.0.0.1:45529/api/v1/namespaces/e2e-tests-kubectl-nlzj2/pods/nginx/exec?command=echo&command=running&command=in&command=container&container=nginx&container=nginx&stderr=true&stdout=true: unexpected EOF
     [] <nil> 0xc4211d3830 exit status 1 <nil> <nil> true [0xc4203e6828 0xc4203e6840 0xc4203e6858] [0xc4203e6828 0xc4203e6840 0xc4203e6858] [0xc4203e6838 0xc4203e6850] [0x11bf750 0x11bf750] 0xc420f2db60 <nil>}:
    Command stdout:
    
    stderr:
    W0810 18:25:44.244920    4112 http.go:342] Error reading backend response: unexpected EOF
    error: error sending request: Post http://127.0.0.1:45529/api/v1/namespaces/e2e-tests-kubectl-nlzj2/pods/nginx/exec?command=echo&command=running&command=in&command=container&container=nginx&container=nginx&stderr=true&stdout=true: unexpected EOF
    
    error:
    exit status 1
    
not to have occurred
/go/src/k8s.io/kubernetes/_output/dockerized/go/src/k8s.io/kubernetes/test/e2e/framework/util.go:2066```