Fix Kotlin Documentation

Author: jzheaux

docs/src/test/kotlin/org/springframework/security/kt/docs/servlet/authentication/servletauthenticationauthentication/CopyAuthoritiesTests.kt

@@ -10,6 +10,7 @@ import org.springframework.security.authentication.TestingAuthenticationToken
 import org.springframework.security.authentication.UsernamePasswordAuthenticationToken
 import org.springframework.security.authentication.ott.OneTimeTokenAuthentication
 import org.springframework.security.core.Authentication
+import org.springframework.security.core.BuildableAuthentication
 import org.springframework.security.core.authority.AuthorityUtils
 import org.springframework.security.core.authority.FactorGrantedAuthority
 import org.springframework.security.core.context.SecurityContextHolder
@@ -28,7 +29,7 @@ class CopyAuthoritiesTests {
         // tag::springSecurity[]
         var latestResult: Authentication = authenticationManager.authenticate(authenticationRequest)
         val previousResult = SecurityContextHolder.getContext().authentication;
-        if (previousResult?.isAuthenticated == true) {
+        if (previousResult?.isAuthenticated == true && latestResult is BuildableAuthentication) {
             latestResult = latestResult.toBuilder().authorities { a ->
                 a.addAll(previousResult.authorities)
             }.build()