Bug: Mode calculation for Gamma distribution

[tessob]

Gamma distribution with the following parametrization:

Gamma::new(0.2422859569300416, 0.0000033637229958478293)

produces really weird mode value – Some(-225260.53542615683).

In Wolfram there is no mode for this distribution

[Orion Yeung]

It's singular at x=0 for α<1, I suppose wolfram choose none in lieu of x=0, I'd prefer x=0