Since fb08d477eb was merged, storlets
invocation on proxy fails because the close method is called even if
iterator has list_iterator type which doesn't have the close method.
This change replaces the direct call of the close method by
close_if_possible, to avoid Exception if the response iterator is
list or tuple, which don't support the close method.
Change-Id: I851cbb17465b5693dda5206aeae0a3191c9fe4f4
Closes-Bug: #1901453
7b5868266a