Update clientAuthService.js

[guitarmanusa]

This option explicitly tells modern browsers to send the client certificate with the XHR request. Without this option the client certificate will not be sent, and guacamole will NOT receive the X-Client-Verified and X-Client-Certificate headers from the reverse proxy and the authentication attempt will fail.

[necouchman]

@guitarmanusa Thanks for this fix - I've approved your Jira account, so as soon as you create the Jira issue and tag the PR and commit with that, we can get this merged (assuming @mike-jumper doesn't have any concerns).