Webserver.Headers config cannot be set via environment variables

[ezk84]

If I have config like:

[webserver]
port = ":9090"
uri_prefix = "${TEGOLA_URI_PREFIX}"

    [webserver.headers]
	Access-Control-Allow-Origin = "${TEGOLA_ALLOW_ORIGIN}"

...

The value isn't picked up from the environment, instead the literal string ${TEGOLA_ALLOW_ORIGIN} is sent as the header value. Other values are correctly picked up from the environment.

[ARolek]

I think this is caused by nested config blocks not supporting the env var parsing: https://github.com/go-spatial/tegola/issues/597