How can i add an HTTP header in my app on App Engine?

I have an Angular app running on App Engine and for security reasons i need to add some headers to server responses, for example the HSTS header. Checking on the App Engine documentation i found that apparently you can do it by adding the following lines in your app.yaml file:

handlers:
- url: /images
  static_dir: static/images
  http_headers:
    X-Foo-Header: foo
    X-Bar-Header: bar value
    vary: Accept-Encoding
  # ...

However, this is not adding headers to my response after adding those lines to my app.yaml file.

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Can you help me to find out what am i doing wrong? I am new on App Engine Configurations.

Thanks in advance