Fix type unstable Uniform

[prbzrg]

this fixes:

julia> dist = Uniform{Float32}(-0.1f0, 0.1f0)
Uniform{Float32}(a=-0.1f0, b=0.1f0)

julia> typeof(rand(dist))
Float64

[devmotion]

See #1796 and other discussions in the repo for why this is inconsistent with the current design of Distributions.