Change SafePoisson and SafeNegativeBinomial to return value from rand calls

[SamuelBrand1]

In light of discussion here JuliaStats/Distributions.jl#1905 (comment)

We can expect the behaviour of rand(d::Distributions) to change in a way that will ameliorate edge-case numerical instability, e.g. InexactErrors. The origin of these issues as I understand it is the behaviour of Distributions.eltype, which can't always precisely predict the output type of a rand call from the type of the Distribution. This tracks the point made by @devmotion on the PR.

The simplest fix I can think of before (possible) implementation of linked PR is to either

• make Safe... Continuous and treat integers as x.0
• make Safe... Real valued.