Support registries not hosted at root

[Yspadadden]

Our Kafka host provides the schema registry under a url like this: https://whatever.srvs.cloudkafka.com/schemaregistry/.
The client creates requests the following way:

return new Request(
        'GET',
        '/subjects',
        ACCEPT_HEADER
    );

As the Guzzle documentation for base_uri states:

When a relative URI is provided to a client, the client will combine the base URI with the relative URI using the rules described in RFC 3986, section 5.2.

This means the request that is sent is something like: https://whatever.srvs.cloudkafka.com/subjects. The path given in the base_uri is lost.

The solution seems to be to remove the leading / from all requests in the this file. I have tested this solution by locally modifying the file inside the vendor directory in my project.