Fix display behavior

Currently,

```
julia> using Interpolations
julia> Interpolation(rand(10), Linear(OnGrid()), ExtrapError())
10-element Interpolation{Float64,1,Linear{OnGrid},ExtrapError}:
 #undef
 #undef
 #undef
 #undef
 #undef
 #undef
 #undef
 #undef
 #undef
 #undef
```

This isn't very pretty... I seem to recall something about this reported and fixed for Grid.jl, but now I can't find it.