Add another test-case for upnp.core.action.
Merge branch 'maintain/0.6.x' into develop
Add another test-case for upnp.core.action.

This one is not in maintain/0.6.x since it is changed behaviour.
Cleanup handling of returned values.

This changes the behaviour: Extraneous result values are silently
ignored now. Former versions would raise an IndexError, which was
not handled anywhere in coherence.
Minor cleanup for handling argument `InstanceID`.

Do not handle it within the loop, which will be removed in the next
commit.
Cleanup check for missing or extraneous arguments.

Use sets instead of looping twice over lists, which are build twice, too.
Cleanup: Action instances always have callback attribute.

Prior to this it got set in set_callback() and required exception-
handling in get_callback().
Cleanup: Rearrange code, prefix private methods with an underscore.
Cleanup: Shortcut a designator by using a variable.
Cleanup: Wrap ugly long lines, remove out-commented code.
Merge branch 'cleanup-action' into develop