Fix promotions with irrationals

[devmotion]

This PR shows what I had in mind when commenting on #258.

Should fix #156 but is quickly typed on my mobile phone and untested.

[prbzrg]

There's a way that already exist:

DistributionsAD.jl/src/multivariate.jl

Lines 198 to 199 in 5847e86

	s = sum(log, d.σ)
	return length(d) * (1 + oftype(s, log2π)) / 2 + s

Thanks for the idea, I updated my PR. my goal is the issue to be fixed; feel free to close my PR.

[devmotion]

I'm currently AFK but I'll finalize the PR (and eg add tests) tonight. Then we should be able to merge and release the fix within the next few days.

[yebai]

Thanks @devmotion!