HTTPCLIENT-1351: Made URICollection a concrete class; added #clear() method

git-svn-id: https://svn.apache.org/repos/asf/httpcomponents/httpclient/trunk@1487448 13f79535-47bb-0310-9956-ffa450edef68
HTTPCLIENT-1359: repeated requests using the same context fail if they redirect
Contributed by James Leigh <james at 3roundstones.com>

git-svn-id: https://svn.apache.org/repos/asf/httpcomponents/httpclient/trunk@1487463 13f79535-47bb-0310-9956-ffa450edef68