Add support for "direct naked impersonation"

[tomqwpl]

Addresses issue #464

Happy if you want to change the method name :-)

[tomqwpl]

Someone will have to help me understand what the lint complaint here is. As far as I can tell it's nothing to do with my code, but the lint output shows nothing in terms of where the error is. The code I added doesn't have "c", "err" or "token". The code in client_test.go contains:

func FailRequest(client *gocloak.GoCloak, err error, failN, skipN int) *gocloak.GoCloak {
	client.RestyClient().OnBeforeRequest(
		func(c *resty.Client, r *resty.Request) error {

That's the only reference to a "c" being an unused parameter I can see. So something appears to have changed in the linting?