issuer URIs with a path part don't work

i originally opened this as https://github.com/solid/oidc-auth-manager/issues/44 but perhaps i should have started here to target the right audience.

oidc-auth-mananger seems to make an invalid assumption that all OIDC Issuers are URIs with an empty path part. if a webid has an OIDC Issuer URI with a path part (like my webid https://zenomt.zenomt.com/card.ttl#me with issuer https://zenomt.com/oidc/), it can never be matched against the id_token iss claim because the discovered issuer URI is reduced to its origin before being compared. see these lines in preferred-provider.js.

OIDC Issuers are allowed to have path parts.

从 src/preferred-provider.js 第 70-73 行附近开始，这里发现的 issuer URI 在与 id_token 的 iss claim 比较之前会被缩短。验证 OpenID Connect 对 issuer 路径部分的要求，然后在匹配时保留路径。当 https://zenomt.com/oidc/ 这样的 issuer 能够与对应的 iss claim 匹配时，即表示完成。