returning NaN caused due to Floating Point Precision Error 

this is a very specific edge case when comparing  the distance of the zip codes 15601 and 15601 
```js
zipcodes.distance(15601, 15601); //expected to be 0
```
But it returns `NaN`  due to the rounding error caused by distance method which passes `1.0000000000000002`  when passed to Math.acos() which returns `NaN`
