I didn't see an appropriate `find` method:

```julia
da = @data [true false; NA true]
find(array(da, false)) # is fine
find(da) # find(A::AbstractArray{T,N}) at array.jl:1231 can't handle it
find(da[:, 1]) # find(dv::AbstractDataArray{Bool,1}) at datavector.jl:17 handles it
```