Unit test dropkey

Adds the ability to drop keys from a vector tile.

Merge after https://github.com/mapbox/vtfx/pull/32
You should check for more than this, otherwise if `options.options` is an empty array, or non-existent, you'll get a TypeError.

maybe something like (there's probably a better way than this tho):
```
if (options.options) throw new Error('Field options not provided');
if (options.options.length > 0 && options.options[0] && !options.options[0].field) throw new Error ('field must be given option required')
```