Add a failing spec

`Lotus::Action::Throwable` overrides `throw` so it's impossible to use
`catch`/`throw` as one would usually in Ruby.
Replace `#throw` override with `#halt` method

Redefining `Kernel#throw` is surprising since it prevents developers
from using `catch`/`throw` pattern inside actions. Instead, we provide
`#halt` method that accepts a status code (and does the `throw`
internally).
Merge pull request #8 from sidonath/restore-throw

Replace `#throw` override with `#halt` method