`UnknownMass` called with bad argument

[cscherrer]

Currently (v0.14.7) we have

massof(m::AbstractMeasure) = UnknownMass(m)

But there's no such method:

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

massof(m::AbstractMeasure) = UnknownMass()