Implement getDataSource() in base class WFSStreamingRequest

Since the following case is very common:

``` r
getDataSource = function() {
    private$getURL()
}
```

This could be implemented in `WFSStreamingRequest` making it a non-abstract method. This `WFSStreaming*` and `WFSCaching*` classes could be used directly (for simple use cases) without the need of implementing subclasses first.
