Make -N/--no-default-middleware option work

This fixes the -N (a.k.a. --no-defaut-middleware) option, which
was not working.  The problem was that Unicorn::Configurator::RACKUP
is cleared before the lambda returned by Unicorn.builder is run,
which means that checking whether the :no_default_middleware option
was set from the lambda could not detect anything.  This patch copies
it to a local variable that won't get clobbered, restoring the feature.

[ew: squashed test commit into the fix, whitespace fixes]

Signed-off-by: Eric Wong <normalperson@yhbt.net>