Re-evaluating parameterized type definitions erros

I noticed this inconsistency (both in 0.4 and 0.5):

``` julia
julia> type B
       a
       end

julia> type B
       a
       end

julia> type A{T}
       a
       end

julia> type A{T}
       a
       end
ERROR: invalid redefinition of constant A
```

Would be nice if the second would not error.
