Bug with is_a?(Class)

I'm getting `is_a?(Class)` to return `true` unexpectedly:

```
struct X
  def initialize(x)
  end
end

S = X
puts S.class
puts S.is_a?(Class)
puts S.class == Class

s = X.new("s")
puts s.class
puts s.is_a?(Class) # This is unexpectedly returning true.
puts s.class == Class # This returns false, as expected, but doesn't work as a type filter.
```
