Conversion to symbolic units doesn't work

The `uexpand` docstring states:

> Expand the symbolic units in a quantity with symbolic dimensions to their base SI form. In other
words, this converts a quantity with AbstractSymbolicDimensions to one with Dimensions. The
opposite of this function is uconvert, for converting to specific symbolic units, or, e.g.,
**convert(Quantity{<:Any,<:AbstractSymbolicDimensions}, q), for assuming SI units as the output
symbols**.

But:

```
julia> convert(DQ.Quantity{<:Any,<:DQ.AbstractSymbolicDimensions}, DQ.u"kg") |> typeof
DynamicQuantities.Quantity{Float64, DynamicQuantities.Dimensions{DynamicQuantities.FixedRational{Int32, 25200}}}
```

So the dimensions have not become symbolic, how can I do this?