HTTPCLIENT-1032: change variant storage mechanism in HttpCacheEntry from a Set to a Map; Set-based interface retained for backwards-compatibility. Currently, the map just maps a variantURI to itself--in essence it is still really just a set; however, this sets us up for mapping different variant keys to the same cache key (coming next).
httpcomponents/httpclient/trunk/httpclient-cache/src/main/java/org/apache/http/client/cache/HttpCacheEntry.java