Implement getDataSource() in base class WFSStreamingRequest

[jlehtoma]

Since the following case is very common:

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.