Dispatch issue

Julia dispatch issue:
Let’s say we have `AFArray <: AbstractArray{T,4}`

now there are methods that are dispatched across `AFArray` and `AFAbstractArray`

``` julia
f1 = sum{T,N}(a::AbstractArray{T<:Any, T<:Any})
f2 = sum{T}(a::AFArray{T})
```

now suppose `a` is an `AFArray`, and I call `sum(a)`, Julia should call `f2`, but doesn't.