unit() function not supporting log units

As per the title, the unit function doesn't work for log units. Tested for dB and V_mag (UnitfulAstro)

```julia
julia> unit(1.0u"dB") == u"dB"
false

julia> unit(1.0u"m") == u"m"
true

julia> unit(1.0u"rad") == u"rad"
true
```

The return is as below. I compared this to radians for another NoDims unit:

```julia
julia> unit(1.0u"dB") |> typeof
Unitful.FreeUnits{(), NoDims, nothing}

julia> unit(1.0u"rad") |> typeof
Unitful.FreeUnits{(rad,), NoDims, nothing}

```