Allow using custom ports in endpoints without using `testing=True`, as discussed in #55 #74

…s discussed in #55: - Remove the check for custom port in endpoint uri validation. - Validate the `issuer` and/or url values before doing any HTTP request in `OAuth2Client.from_discovery_endpoint()`, unless `testing=True` is used. - if `testing=True`, don't try to validate the `jwks_uri` in `OAuth2Client.from_discovery_document()`

…t}`.