url-equivalent issuer comparisons

[cainlevy]

The URLs https://example.com and https://example.com:443/ can be considered equivalent issuers. The keratin/authn-rb library accomplishes this by coercing to URLs for comparison, but go's net/url library makes no assumptions (per golang convention).

Supporting this lenient issuer matching will take a bit more effort. It is not necessary for the library to function when configured precisely, though.