ChangeSet returns null for Pipeline Jobs

We've come across an inconsistency in the Jenkins API for freestyle jobs vs pipeline jobs due to an issue raised on one of our projects TransFICC/transficc-feedback#4

The Jenkins API returns ```changetSet``` for a freestyle job build whilst for a pipeline job build it returns ```changeSets```. This means I can't get the change set information for a pipeline job unfortunately.

I am going to see if there is a nice way of being able to deal with this inconsistency in the library, but this ultimately feels like a Jenkins bug to me.

Start by tracing how the Java client reads Jenkins build change-set data, comparing handling of the freestyle `changetSet` field with the pipeline `changeSets` field described here. Done means pipeline job builds expose their change-set information instead of returning null; add or run a focused regression test if the repository has coverage for build parsing.

索引モデルが issue の本文から書いたものです。