Fix workflow array variable deserialization on REST APIs

In the Choose Participants task for the parallel review, the participants/assignees must be supplied as a quoted array ("[/"user/"]"), but if you make the mistake of passing it as a regular array (["user"]), it won't issue an error.

Instead, the server proceeds, but when you request the next task, there are none to be found for that worklfow. Debugging this can be time consuming.

Ideally, we should not have to pass a quoted array ("[/"user/"]") but an true array.

This behavior has been coded from the beginning in the whole stack, see:

• org.nuxeo.ecm.platform.routing.core.impl.GraphVariablesUtil.setJSONVariables(DocumentModel, String, Map<String, String>, boolean)
• org.nuxeo.ecm.automation.core.util.DocumentHelper.setProperty(CoreSession, DocumentModel, String, String, boolean)

Note that it affects both the REST API and the automation api (/nuxeo-routing-core/src/main/java/org/nuxeo/ecm/platform/routing/core/api/operation/StartWorkflowOperation.java)