Fix a minor bug in jlib:iq_to_xml/1.

We know from source code: 
```
            %% The iq record is a bit strange.  The sub_el field is an
            %% XML tuple for requests, but a list of XML tuples for
            %% responses.
```

This PR changes `iq_to_xml/1` according to `iq_info_internal/1`.
Output before:

```
{xmlelement,<<"iq">>,
[{<<"id">>,<<"c3a5e0900a1162db7b7229aba1e48107">>},{<<"type">>,<<"get">>}],
{xmlelement,<<"query">>,[{<<"xmlns">>,<<"urn:xmpp:mam:tmp">>}],[]}}
```

Output after:
```
{xmlelement,<<"iq">>,
[{<<"id">>,<<"c3a5e0900a1162db7b7229aba1e48107">>},{<<"type">>,<<"get">>}],
[{xmlelement,<<"query">>,[{<<"xmlns">>,<<"urn:xmpp:mam:tmp">>}],[]}]}
```