Fix beta function for BigFloats

Since `beta(Float64/32)` calls `lgamma_r` which is not defined for `BigFloat`, a call to `beta` with a `BigFloat` gave a stack overflow. Fixing `beta(BigFloat)` shows that we might want a more precise `beta(Float64)` at some point
```julia
float64(beta(1000,0.2)/beta(BigFloat(1000),BigFloat(0.2))-1)
-8.83435934881076e-13
```