XYZ layer type throws Javascript TypeError exception

There appears to be a naming inconsistency with the XYZ layer configuration's url property.

The Javascript (plugins/layer_types/openlayers_layer_type_xyz.js) references a property called: options.url. Whereas the form that creates XYZ layers (plugins/layer_types/openlayers_layer_type_xyz.inc) saves the property $form['base_url'] which leads to a options.base_url value in Javascript.

To confuse things a little further, the default XYZ layers in (includes/openlayers.layers.inc) define a 'url' value but if these layers are cloned, edited and exported, they end up with 'base_url' values. These then break as the Javascript checks for options.url.

Attached a patch that changes the XYZ layer form element name from 'base_url' to 'url' to avoid having to update all the default XYZ layers.