beta function for complex arguments

[stevengj]

beta(x,y) does not currently support complex arguments, because it calls the Base.Math.lgamma_r function, which only supports real arguments, rather than lgamma, which takes complex arguments.

Seems like an easy fix to extend lgamma_r to complex arguments. Since lgamma for complex arguments does not take an absolute value, it seems like we could just define a fallback lgamma_r(z::Number) = lgamma(z), 1

This also relevant to #14165.