The JSON object the webhook sends contains this for the Sprint customfield:

"customfield_10020": [
        "com.atlassian.greenhopper.service.sprint.Sprint@4ccd3f[id=2,rapidViewId=6,state=FUTURE,name=TIB Sprint 2,goal=<null>,startDate=<null>,endDate=<null>,completeDate=<null>,sequence=2]"
      ],

Looks like it's just printing the toString() of the Java object instead of serialising it to a proper JSON object. Would be good if this return a JSON object.