lowercase index names from URL

[jeremybmerrill]

elasticsearch requires index names to be lowercase

URLs, generally, are case-insensitive.

so, when we get the index name from the URL, we should downcase it to match what elasticsearch is expecting (and so users still can rely on case-insensitive URLs) -- so /search/MYINDEX is the same as /search/myindex, both mapping to the myindex index in elasticsearch