Credentials are not checked

[fleaz]

Hey,
while playing around with this library I discovered that wrong credentials are silently accepted and the functions (like Tests().All()) just return an empty array.

It would be great if directly when creating a new client object, the credentials are checked against the Statuscake API to immediately raise some kind of InvalidCredentials exception. Assuming that there is some endpoint in the Statuscake API which can be used for this check...

Best regards,
Felix