I’m getting `NaN`

from a multinomial distribution, where I expected 1. Is this a bug or am I doing something wrong?

```
using Distributions
dist = Multinomial(2, [0.999,0.001])
pdf(dist,[2,0]) ## 0.998, ok
dist = Multinomial(2, [1.0,0.0])
pdf(dist,[2,0]) #NaN, not ok
```

The equivalent limit of the binomial has no problem:

```
pdf(Binomial(1,0.999),[0,0.5,1]) ## [0.001, 0.0, 0.999]
pdf(Binomial(1,1.0),[0,0.5,1]) ## [0.0, 0.0, 1.0]
```