Add support for empty geocoding Task constructors.

as mentioned in my #49 comment.  it seems helpful to just assume people want to use the World Geocoding Service so that developers can just instantiate a Task object with an empty constructor.

ie:
`L.esri.Geocoding.Tasks.geocode()`

made code change and added a relevant test.  
also updated a reused variable in GeoSearch.js to make jshint happy.

let me know if this is unwanted or if there is a more desirable approach.  if its all good, i can update the doc as well.
without the initial `if` statement, a call to `_geocode()` from a control with a custom provider was incorrectly handled as if none had been specified.

since no parameters are included when triggering the instantiation [here](https://github.com/Esri/esri-leaflet-geocoder/blob/master/src/Controls/Geosearch.js#L39), i couldn't think of a better way to check than interrogating within the Task instantiation.

happy to pursue something less ugly.  i have a test case.  if all else fails we can chat about it at the hackathon on friday.