make copy correctly handle 0-dimensional SubArray (#39809)
* make copy correctly handle 0-dimensional SubArray
The current definition of `copy` for `SubArray` [here](https://github.com/JuliaLang/julia/blob/master/base/subarray.jl#L70) has the following consequence:
```
julia> x = [1]
1-element Array{Int64,1}:
1
julia> y = @view x[1]
0-dimensional view(::Array{Int64,1}, 1) with eltype Int64:
1
julia> copy(y)
1
```
which is inconsistent with the contract for `copy` that promises to produce an array when array is copied, e.g.:
```
julia> x = fill(1)
0-dimensional Array{Int64,0}:
1
julia> copy(x)
0-dimensional Array{Int64,0}:
1
```
