Type of sample of `Exponential{T}` is not `T`

[wouterwln]

MWE:

θ = 1.0f0
d = Exponential(θ)
typeof(rand(Exponential(θ))) == typeof(θ)

returns false

Because Exponential sampling is also used when sampling Gamma with shape 1, the problem also occurs there. For Gamma this introduces a type instability, since the return type for sampling for a Gamma depends on whether or not the alpha parameter is equal to 1.0.