mean(f, x, weights) not supported

This method is not supported. Could it be?

```jl
julia> mean(==(1), [1,2,3])
0.3333333333333333

julia> mean([1,2,3], weights([10,20,30]))
2.3333333333333335

julia> mean(==(1), [1,2,3], weights([10,20,30]))
ERROR: MethodError: no method matching mean(::Base.Fix2{typeof(==), Int64}, ::Vector{Int64}, ::Weights{Int64, Int64, Vector{Int64}})
```

