Recently when running
set apphandler => 'PSGI'
it did not pick up because setting the apphandler affected only
the Dancer App, while the apphandler was (correctly) decided and
used by the Dancer Runner.
By making the "apphandler" option a global trigger that updates the
Dancer Runner object, this will now work.
Added a test that uses LWP::Protocol::PSGI, which exposed this
problem in GH #447.