fix: trim strict-transport-security header

[LucasDemea]

Fixes #1477.

[swalkinshaw]

Thanks @LucasDemea

[mariusmateoc]

Hi @swalkinshaw @LucasDemea,

If I didn't set hsts_include_subdomains: true I get invalid header strict-transport-security: max-age=31536000; ; (note double "; ;").

I don't know why reject('none') doesn't reject false/empty. I fix it with the following code:

add_header Strict-Transport-Security "max-age={{ [hsts_max_age, hsts_include_subdomains, hsts_preload] | reject('none') | reject('equalto','') | join('; ') | trim }}";