check for described_class before invoking subject

I am opening this pull request as I noticed that if you have some specs using the format below, when you invoke subject, rspec will grab use the ```implicit_subject``` which tries to instantiate the described class.

```ruby
class ClassA
  def initialize(an_arg)
  end
end

describe ClassA do
  it { should be_a Class }
end
```

I have added what I think might be a "hack" fix to this issue. In looking at the commit (sha c1a3644a) I am unsure what the use case was for this feature so I'm not sure how to best accurately address the issue.

This fix may actually be valid but at least I wanted to open up the discussion around this.
Merge pull request #93 from danshultz/patch-1

check for described_class before invoking subject