UnitWeights don't have values to return from dataids

[rofinn]

Current fallback can cause errors like this:

  type UnitWeights has no field values
  Stacktrace:
    [1] getproperty(x::StatsBase.UnitWeights{Int64}, f::Symbol)
      @ Base ./Base.jl:33
    [2] dataids(wv::StatsBase.UnitWeights{Int64})
      @ StatsBase ~/.julia/packages/StatsBase/n494Y/src/weights.jl:26
    [3] dataids(A::SubArray{Int64, 1, StatsBase.UnitWeights{Int64}, Tuple{Vector{Int64}}, false})
    ```

[nalimilan]

Thanks! UnitWeights is indeed always special. Could you add a test?

[rofinn]

The issue is that it’s hard to reproduce the convert path without making a bunch of assumptions about Julia’s type inference. I could add an explicit test, but that doesn’t seem very useful?

[nalimilan]

A basic test is better than nothing. Anyway we shouldn't add lines that are not covered by tests, to ensure that at least they run without errors.

You could also call Base.mightalias (as I did at #793), as IIRC it calls Base.dataids.

[nalimilan]

Can you tag a new release?