Fix JSON generation of Nuxeo signature operations in Explorer

Steps to reproduce the error:

1. Go to http://explorer.nuxeo.com/nuxeo/site/distribution/Nuxeo%20DM-7.10/viewOperation/Services.SignPDF
2. Clic on "JSON definition generation" button
3. A new window open with this exception

   {"entity-type":"exception","code":"javax.ws.rs.WebApplicationException","status":500,"message":null}