promotion between SampleBufs and Arrays fails

[ssfrr]

MWE:

julia> promote(SampleBuf(randn(10), 48000), randn(10))
ERROR: promotion of types SampleBuf{Float64,1} and Array{Float64,1} failed to change any arguments
Stacktrace:
 [1] sametype_error(::Tuple{SampleBuf{Float64,1},Array{Float64,1}}) at ./promotion.jl:306
 [2] not_sametype(::Tuple{SampleBuf{Float64,1},Array{Float64,1}}, ::Tuple{SampleBuf{Float64,1},Array{Float64,1}}) at ./promotion.jl:300
 [3] promote(::SampleBuf{Float64,1}, ::Array{Float64,1}) at ./promotion.jl:283
 [4] top-level scope at none:0