fix rake tasks for rails engine

[senid231]

No description provided.

[grosser]

could you describe the usecase / the error seen a bit more ?

[senid231]

@grosser sure. It's just a draft yet.

Usecase is when you are using parallel_tests for RSpec test on rails engine.
When you developing rails engine you don't have rake tasks like db:migrate, db:structure:load and so on - but you have dummy rails application which have default rails rake tasks namespaced in app:.

I saw that @jimryan already add support of rails engine namespace for db:purge rails tasks in #566
but he doesn't solve the problem completely.

Currently this code tested locally on my project. I want to add some tests for it and fix failing one