copy in constructor

[jariji]

Currently the ROCArray constructor returns its argument unlike Array's constructor. This change uses copy instead.

julia> let x = rand(5)
           y = Array(x)
           z = Array(y)
           y === z
       end
false

julia> let x = rand(5)
           y = ROCArray(x)
           z = ROCArray(y)
           y === z
       end
true

[maleadt]

If we want this, which I assume we do, it may be good to add this as a test to GPUArrays.jl.