Duplicated strict-transport-security header

[aledbf]

NGINX Ingress controller version: 0.26.0 and 0.26.1
Kubernetes version (use kubectl version):

Client Version: version.Info{Major:"1", Minor:"14", GitVersion:"v1.14.2", GitCommit:"66049e3b21efe110454d67df4fa62b08ea79a19b", GitTreeState:"clean", BuildDate:"2019-05-16T16:23:09Z", GoVersion:"go1.12.5", Compiler:"gc", Platform:"linux/amd64"}
Server Version: version.Info{Major:"1", Minor:"16", GitVersion:"v1.16.2", GitCommit:"c97fe5036ef3df2967d086711e6c0c405941e14b", GitTreeState:"clean", BuildDate:"2019-10-15T19:09:08Z", GoVersion:"go1.12.10", Compiler:"gc", Platform:"linux/amd64"}

Environment:

• Cloud provider or hardware configuration: AWS
• OS (e.g. from /etc/os-release):

NAME="Flatcar Linux by Kinvolk"
VERSION=2247.5.0
PRETTY_NAME="Flatcar Linux by Kinvolk 2247.5.0 (Rhyolite)"
HOME_URL="https://flatcar-linux.org/"

• Kernel (e.g. uname -a): Linux 4.19.78-flatcar
• Install tools: Typhoon

What happened:

PR #4601 introduced a duplication in the header strict-transport-security. The behavior is no the same as before when more_set_headers was used.

What you expected to happen:

Only one strict-transport-security header

How to reproduce it (as minimally and precisely as possible):

Any Ingress with TLS correctly configured and a service that also returns the header strict-transport-security returns two instead of just one instance of the header.

[aledbf]

/assign @ElvinEfendi