Small fixes for density estimation

Querying for the density at a bin centered at zero no longer returns `NaN`.  For histograms that have only seen one value, querying the density at that point will return `Infinity`.  This, I believe, is the correct result for a histogram whose CDF is essentially a step function.

```clojure
(-> (create)
    (insert! 0)
    (density 0))
;; Infinity
```

Issue #38