Fixed directions unitSystem option rendering

When the `$this->directions->request_options['units']` option is defined this code rendered 
```javascript
unitSystem: google.maps.DirectionsUnitSystem.1
```
javascription code. 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`.
Merge pull request #1 from e1himself/patch-1

Fixed directions unitSystem option rendering