Fix HttpHandler did not use ServerCertificateValidationCallback

[kaedei]

The Client class have definitions of ServerCertificateValidationCallback and ServerCertificateValidation(), but never use them in the internal HttpClientHandler.

This PR fix it, user now can use:

var client = new WebDAVClient.Client();
client.ServerCertificateValidationCallback = (sender, certificate, chain, sslPolicyErrors) => true;

to customize their own validation codes when connecting to some self-signed certificate WebDAV servers.

Also, this PR pulls ServerCertificateValidationCallback up to IClient interface.