authPrompt: set next button to next when asking for username

If the next button ever gets set to Sign In, it won't
get reset to next until the next question asked by pam.

This commit ensures it gets reset to Next when asking
for the username.

https://bugzilla.gnome.org/show_bug.cgi?id=752739

js/gdm/authPrompt.js

@@ -432,6 +432,7 @@ const AuthPrompt = new Lang.Class({
         let oldStatus = this.verificationStatus;
         this.verificationStatus = AuthPromptStatus.NOT_VERIFYING;
         this.cancelButton.reactive = true;
+        this.nextButton.label = _("Next");
 
         if (this._userVerifier)
             this._userVerifier.cancel();