Specs for where $_ is defined and when it is defined?

Why is this method necessary? We should strive to use as little machinery as possible to demonstrate a behavior. Wouldn't this work fine?

```ruby
it "is nil by default" do
  proc { $_ }.call.should be_nil
end

it "is defined as a global variable" do
  proc { defined?($_) }.call.should == "global-variable"
end
```
This illustrates that each facet of behavior should be independently stated, which allows a useful granularity for iterating on implementation. It also avoids one behavior hiding another when searching for a bug.

Another aspect to consider here is whether the `defined?` result should be considered a bug. It's not really a global variable if it is frame local (ie if it's value depends on the frame you're in).