Fix Task marshaller when workflow schema prefix is different from the name

In the method TaskCompletionRequestJsonReader#getVariables, the workflow properties are written with this syntax

variable.put(property.getName().substring(schemaName.length() + 1), property.getValue());

as you can see, the key is based on the length of the workflow schema name, but the property name is built from the schema prefix. So if the schema name length is different from the schema prefix length, an incorrect key will be used and the Task widget in the UI won't be able to read the workflow properties.

A workaround has been suggested to use the same value for the schema name and prefix, but a fix would be a better long-term solution.