Fix for crash on ask dialog

This failed from e6eac8680fdd100dd15103b7ffcf114ff6bf1784 which was
rubocop introduced to turn an ivar access into an attr_reader.
Unfortunately this was within our nested `Shoes.app` call, so the
scoping is weird and `attr_reader` is actually undefined there.

Stepping into the singleton class lets us use `attr_reader` properly,
without rubocop complaining.
Restore to simple instance variable accessor

Simpler, and disabling the Rubocop rule we don't want that caused it.
Test that ask dialog actually runs

Since it's implemented in Shoes by Shoes, deserves to actually run in
the tests.
Merge pull request #984 from shoes/ask-dialog-crash

Fix crash on ask dialog

* fixes #983 
* specs juchay!