ClientRegistrationsTests assertions should compute the hostname from the issuer

[jzheaux]

ClientRegistrationsTests asserts the following

MockWebServer server;
// ...
assertThat(registration.getRegistrationId()).isEqualTo(this.server.getHostName());

This works coincidentally when the hostname is all lower-case. However, due to a nuance between how MockWebServer computes the hostname and the URL, this check fails when the hostname has mixed case.

Instead, the test should compute the hostname from the issuer, which is the same thing that ClientRegistrations does when determining the default registrationId:

assertThat(registration.getRegistrationId()).isEqualTo(URI.create(this.issuer).getHost());