Set sane defaults for configuration

[trietsch]

Default values could be set as follows (discussed with @Rachnerd). In the constructor of the Client, defaults could be set using:

    const {
      authUrl = 'url',
      ...other
    } = config;

    this.config = other;

This overrides the config as provided in the constructor, by specifying values for properties in the Config interface, that are nullable. So it's important to update the Config interface, and to make the properties nullable. If a user of the driver provides values for e.g. authUrl, then that value will be used. Otherwise, the ones provided in the const are used.