Need more exhaustive check to determine if an HSTS header will be ignored

# 🐛 Bug Report

Consider the function `hsts_check()` in `pshtt/pshtt.py`. It's not just a bad hostname that should cause the HSTS header to be ignored. It is also ignored if the certificate is untrusted, for example. (See item 2 in [the RFC](https://tools.ietf.org/html/rfc6797#section-2.2).) Therefore I think that this `if` clause should be fleshed out a little more.

We should be a little careful when making this change, since [as @echudow pointed out](https://github.com/cisagov/pshtt/pull/193/files/0ec75ba3fcabffa43b5c265d92c3404fc77e0a56#r315338012) in a separate pull request:

> Currently we don't check that the certificate is trusted even on the initial response, so that change could have more negative effects on the results.