Custom units having unexpected effects on results

I'm encountering an issue where custom units are changing how expressions that do not include the new unit are evaluated.

Example:

```
var result = math.eval('1 km / hour') //answer is 1 km/hour
math.createUnit('mph', {definition: '1.0 mile/hour'})
result = math.eval('1 km / hour') //answer is now 0.6213711922373341 mph
```