IMPROVEMENT: Set the last modified attribute of files downloaded from repositories (IVY-823)

Files:
modified: /ant/ivy/core/trunk/src/java/org/apache/ivy/util/url/HttpClientHandler.java (try)
modified: /ant/ivy/core/trunk/CHANGES.txt (try)
modified: /ant/ivy/core/trunk/src/java/org/apache/ivy/util/url/BasicURLHandler.java (try)