`UnknownMass` called with bad argument

Currently (v0.14.7) we have
```julia
massof(m::AbstractMeasure) = UnknownMass(m)
```

But there's no such method:
```julia
julia> methods(MeasureBase.UnknownMass)
# 4 methods for type constructor:
[1] MeasureBase.UnknownMass()
@ ~/git/MeasureBase.jl/src/mass-interface.jl:19
[2] (::Type{T})(x::AbstractChar) where T<:Union{AbstractChar, Number}
@ Base char.jl:50
[3] (::Type{T})(x::T) where T<:Number
@ Core boot.jl:792
[4] (::Type{T})(x::Base.TwicePrecision) where T<:Number
@ Base twiceprecision.jl:266
```

I think this should be
```julia
massof(m::AbstractMeasure) = UnknownMass()
```