Fix the product excerpt not to return the additional product details.

Fix for #330

Addition product information is stored in the post excerpt DB column which means it is returned as the excerpt by default in themes which is not the desired/expected behaviour.

This fix intercepts the 'get_the_excerpt' filter early and returns an empty string which forces WordPress to create an excerpt from the post content in the same way that an excerpt would be created for a post if no excerpt was supplied.