Value handlers should not set values that are for channels not associated to the product

[LucaGallinari]

All value handlers does not check the "scope" information of the values retrieved from Akeneo. It should do this in order to set values on the product only if these are set on a channel that is associated to the product.
For example, if we have two channels on Akeneo, one for the printed catalog and one for the ecommerce, we can set different values for the same attribute for each channel. By the way, the value handler that will import this value, should ignore the value set for the printed catalog and use the one for the ecommerce only.