Merge pull request #1636 from SimonCropp/fix-incorrect-as-usage-in-Ge…

…tAuthenticationTimeEpoch

fix incorrect as usage in GetAuthenticationTimeEpoch

src/IdentityServer/Extensions/PrincipalExtensions.cs

@@ -46,7 +46,7 @@ public static long GetAuthenticationTimeEpoch(this IPrincipal principal)
     [DebuggerStepThrough]
     public static long GetAuthenticationTimeEpoch(this IIdentity identity)
     {
-        var id = identity as ClaimsIdentity;
+        var id = (ClaimsIdentity)identity;
         var claim = id.FindFirst(JwtClaimTypes.AuthenticationTime);
 
         if (claim == null) throw new InvalidOperationException("auth_time is missing.");