fixed #603: not all zero_wrapper define a get() method, which breaks logging in some cases

[devzeb]

Problem:

#603 demonstrates the problem.
A compile error occurs when trying to use a logger in a statemachine, that uses both a process_queue and a member function pointer as an action.

The error message states:
'get': is not a member of 'boost::ext::sml::v1_1_9::aux::zero_wrapper<T,void>'

Solution:

I implemented the member function get() for all specializations of zero_wrapper.
Also I added a test for this use case in policies_logging.cpp.

Issue: #603

[kris-jusiak]

Good catch, thank you @devzeb