Fix getArgumentAt in SISISTest

After update of Mockito from 1.10.19 to 3.8.0 (see 11833f3)
testLoadPages did no longer compile as getArgumentAt was removed in Mockito 2.7.6 (see
mockito/mockito#945). Fixed by replacing with getArgument.

opacclient/libopac/src/test/java/de/geeksfactory/opacclient/apis/SISISTest.java

@@ -48,7 +48,7 @@ public void testLoadPages() throws IOException, OpacApi.OpacErrorException, JSON
 
         doAnswer(invocation -> {
             String accountBase = "https://opac.erfurt.de/webOPACClient/userAccount.do?";
-            String url = invocation.getArgumentAt(0, String.class);
+            String url = invocation.getArgument(0);
             if (url.equals(accountBase + "methodToCall=showAccount&typ=1")) {
                 return html;
             } else if (url