expect non-nil values from #execute
work around crazy block-level ruby 2 bug

turns out that when you call defined?(super) inside a block inside a method,
Ruby 2 returns nil where Ruby 1.9.3 used to return "super". this means that
both the implementation of Makara and the functionality provided by RSpec's
#and_call_original stop working inside the #with_makara method. We fixed it
by completely unrolling both of those block-based methods, inside Makara
itself and inside the specs.

For more information on the Ruby 2 issue and a reduced example, see also
https://gist.github.com/indirect/5514723