MavenModuleSet should always be saved

Corrects a regression in 2.6 introduced by my own #26 but for which there had been no test.

Generally, I would suggest you just delete the `saveItemGroupConfiguration` option and always save folder configurations; I cannot think of any reason why you would want this to be off, much less off _by default_. I would also suggest you delete the `saveModuleConfiguration` option and not save configuration of any `Item` which is not a `TopLevelItem`; or at least not save this by default. (By checking for `TopLevelItem` rather than explicitly for `MavenModule` you should be able to remove the dependency on `maven-plugin` as requested in [JENKINS-19141](https://issues.jenkins-ci.org/browse/JENKINS-19141), making the code more generic and so properly handling matrix configurations, Literate branches and environments, ad nauseam.) If you agree with this proposal I may have time to draft it as a PR.