Fix #7 - Error upload might be deleted from rbtree

Due to the done flag not being correctly set to 0 when the error
tracker triggers, it could happen that such upload could be mistakenly
removed from the the rbtree that contains all in-flight uploads.
Thanks to Theo Cushion for finding this issue.

Signed-off-by: Brice Figureau <brice@daysofwonder.com>

ngx_http_uploadprogress_module.c

         node->key = hash;
         up->len = (u_char) id->len;
         up->err_status = r->err_status;
+        up->done = 0;
+        up->rest = 0;
+        up->length = 0;
+        up->timeout = 0;
+
         ngx_memcpy(up->data, id->data, id->len);
 
         up->next = ctx->list_head.next;