#13 Switch to update mode when creating attributes of existing item

[Björn Tantau]

An empty manufacturer attribute of an existing product where the attribute was previously not empty did not get deleted when the product was updated in create mode.
The attached patch adds a $_current_mode property on Magmi_ProductImportEngine and is switched to "update" if an existing item is updated, even if the importer runs in "create" or "xcreate" mode.