Fix for commands returning DevVar(Long|Double)StringArray

This should fix running commands with return type DevVarLongStringArray or DevVarLongStringArray. I don't know of any other types that have this peculiar form, see https://tango-controls.readthedocs.io/projects/rfc/en/latest/9/DataTypes.html#structures for more info.

This could be the cause of #12.