Fix conversion to unitless Quantity

[sostock]

Unitless Quantitys can be created by calling the appropriate constructor, but a convert call currently fails:

julia> Quantity{Int, NoDims, typeof(NoUnits)}(5)
5

julia> typeof(ans)
Quantity{Int64, NoDims, Unitful.FreeUnits{(), NoDims, nothing}}

julia> convert(Quantity{Int, NoDims, typeof(NoUnits)}, 5)
ERROR: type Int64 has no field val
[…]

This PR fixes the convert implementation to allow conversion to Quantity{T, NoDims, typeof(NoUnits)}.