Softmax fails on array of TrackedReal

I first noticed this problem with Flux 0.8.2, but it may have existed in earlier versions.

```
using Flux
tracked_array = Flux.param(Flux.glorot_uniform(10,3)); # tracked array of reals
Flux.softmax(tracked_array) # all good

array_of_tracked_reals = reduce(hcat, [tracked_array,tracked_array]);
Flux.softmax(array_of_tracked_reals) # fails with MethodError: no method matching softmax!(...)
```
I can get around it using hcat(...) #with splats, which creates a tracked_array of reals, but I would also like to use the array of tracked reals for other reasons.