Add X-total-count in response header from server

[ansor4]

Return the total count in a new header x-total-count.
The number returned should reflect the filters passed in the query. For example, if there are 13 items in artworks_data that has dominant_colors: "brown", then retrieve({dominant_colors: ["brown"]}) should return with a header x-total-count: 13

This will allow the candidate to correctly know the last page without looking at the data fixture in server.rb.