fix bug (?) :-)

previously, if you called curl_easy_perform and then set the global dns
cache, the global cache wouldn't be used.  I don't see this really happening
in practice, but this code allows you to do it.

lib/easy.c

@@ -233,14 +233,16 @@ CURLcode curl_easy_perform(CURL *curl)
 {
   struct SessionHandle *data = (struct SessionHandle *)curl;
 
-  if (!data->hostcache) {
-    if (Curl_global_host_cache_use(data)) {
+  if (Curl_global_host_cache_use(data) && data->hostcache != Curl_global_host_cache_get()) {
+    if (data->hostcache) {
+      Curl_hash_destroy(data->hostcache);
+    }
     data->hostcache = Curl_global_host_cache_get();
   }
-    else {
+
+  if (!data->hostcache) {
     data->hostcache = Curl_hash_alloc(7, Curl_freednsinfo);
   }
-  }
   
   return Curl_perform(data);
 }