fix test logic

Author: lbalmaceda

src/test/java/com/auth0/client/mgmt/ManagementAPITest.java

@@ -77,9 +77,11 @@ public void shouldThrowWhenApiTokenIsNull() throws Exception {
 
     @Test
     public void shouldThrowOnUpdateWhenApiTokenIsNull() throws Exception {
+        ManagementAPI api = new ManagementAPI(DOMAIN, API_TOKEN);
+
         exception.expect(IllegalArgumentException.class);
         exception.expectMessage("'api token' cannot be null!");
-        new ManagementAPI(DOMAIN, null);
+        api.setApiToken(null);
     }
 
     @Test