`distribute` no longer works on ranges

I believe this used to work, but now gives
```
julia> distribute(1:100)
MethodError: no method matching UnitRange{Int64}(::Array{Int64,1})
Closest candidates are:
UnitRange{Int64}(::Any, !Matched::Any) where T<:Real at range.jl:255
UnitRange{Int64}(!Matched::UnitRange{T<:Real}) where T<:Real at range.jl:846
UnitRange{Int64}(!Matched::UnitRange) where T<:Real at range.jl:847
...

Stacktrace:
[1] empty_localpart(::Type, ::Int64, ::Type) at /home/jeff/.julia/packages/DistributedArrays/XV7NS/src/darray.jl:66
[2] macro expansion at ./task.jl:264 [inlined]
[3] macro expansion at /home/jeff/.julia/packages/DistributedArrays/XV7NS/src/darray.jl:84 [inlined]
[4] macro expansion at ./task.jl:244 [inlined]
[5] DArray(::Tuple{Int64,Int64}, ::Function, ::Tuple{Int64}, ::Array{Int64,1}, ::Array{Tuple{UnitRange{Int64}},1}, ::Array{Array{Int64,1},1}) at /home/jeff/.julia/packages/DistributedArrays/XV7NS/src/darray.jl:82
[6] DArray(::Function, ::Tuple{Int64}, ::Array{Int64,1}, ::Array{Int64,1}) at /home/jeff/.julia/packages/DistributedArrays/XV7NS/src/darray.jl:169
[7] #distribute#61(::Array{Int64,1}, ::Array{Int64,1}, ::Function, ::UnitRange{Int64}) at /home/jeff/.julia/packages/DistributedArrays/XV7NS/src/darray.jl:501
[8] distribute(::UnitRange{Int64}) at /home/jeff/.julia/packages/DistributedArrays/XV7NS/src/darray.jl:494
[9] top-level scope at In[18]:1
```

使用 `distribute(1:100)` 重现失败，并检查 `src/darray.jl`，尤其是第 66 行附近的 `empty_localpart` 以及第 494-501 行附近的 `distribute` 方法。将失败的 range 路径与 stack trace 进行比较，并验证分发 `UnitRange` 时可以完成，且不会出现报告的 `MethodError`。

由索引模型根据 Issue 内容生成。