This commit fixes the `@ConditionalOnClass` typo on the GsonTester
FactoryBean, which was testing for `ObjectMapper` instead of `Gson`.

This means that the next condition `@ConditionalOnBean(Gson.class)`
throws a `ClassNotFoundException: com.google.gson.Gson` since it's
trying to load that class to check for an existing Bean of that type.

Fixes gh-6502