Fixed directions unitSystem option rendering

When the `$this->directions->request_options['units']` option is defined `getMapJS` method rendered this javascript code:
```javascript
{
unitSystem: google.maps.DirectionsUnitSystem.1
}
```
Obviously this is wrong.

Also 'imperial' and 'metric' string literals should be uppercase. Because `google.maps.DirectionsUnitSystem` contains `IMPERIAL` and `METRIC` constants but not `imperial` and `metric`.