On product search include post_excerpt

When adding a product,  WC made the design decision to map the 'Product Short Description' to the post_excerpt field. However WC fails to include the post_excerpt field in any search results. (Wordpress does not make this easy)

This patch modifies the search query (only during is_search() and during a WC shop loop) to include post_excerpt in the query.

This allows the WC Product Search widget to actually returns results based on text entered in the Product Short Description.