`uconvertp`/`uconvertrp` fail to convert dimensionless quantities to `Gain`

[sostock]

The following should return -60dB, since 1mm/m == 0.001:

julia> uconvertrp(u"dB", 1.0u"mm/m")
ERROR: 1.0 is not obviously a ratio of power or root-power quantities; use `uconvertrp` or `uconvertp` instead.
[…]

The error message is even saying that one should use uconvertrp or uconvertp, even though that is what was used.