[HttpFoundation] Fixed isSecure() check to be compliant with the docs

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  |no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

According to the [php docs](http://www.php.net/manual/en/reserved.variables.server.php) the `HTTPS` field will be non-empty when the query was issued via HTTPS.

It isn't restricted to only "on" and 1.
Exception: "off" is sent by IIS

BC breaks: no, because old behavior was not conform with the docs.