fix error: Can't set more than one tier_price with api

Currently is it not possible to set multiple tier prices with the api.
If you use ```catalog_product_attribute_tier_price.info``` you get all tier_prices (more than one if you have set more than one in the magento administation) but you can set only one with ```catalog_product_attribute_tier_price.update``` and when you do that the other tier_prices are deleted.

With my patch it is possible to set multiple tier-prices.