Fix connection reuse with IDN host names #592
Always use the ACE form of IDN hostnames as key in the connection cache.
Previously, curl created multiple connections when following redirections from a server with an IDN hostname, e.g. http://äöü.example.com/page1 --> http://äöü.example.com/page2 . The Unicode hostname was used as a key for the connection cache, and for the second request, the ACE name was used to search an existing connection. Therefore curl did not find the existing connection.