always close pool connections to avoid zlib warnings

previously, we only closed connections when there are no errors.
when we hit errors while executing, we saw misleading zlib errors where instead it was just the ssh command failing.
so lets always close connections to avoid those errors.

lib/sshkit/backends/netssh.rb

@@ -79,10 +79,9 @@ def download!(remote, local=nil, options = {})
 
       # Note that this pool must be explicitly closed before Ruby exits to
       # ensure the underlying IO objects are properly cleaned up. We register an
-      # at_exit handler to do this automatically, as long as Ruby is exiting
-      # cleanly (i.e. without an exception).
+      # at_exit handler to do this automatically.
       @pool = SSHKit::Backend::ConnectionPool.new
-      at_exit { @pool.close_connections if @pool && !$ERROR_INFO }
+      at_exit { @pool.close_connections if @pool }
 
       class << self
         attr_accessor :pool