Utilize exported function information to avoid undef thrown

Elapsed time to export XML of one node with 1000 child elements are

Before: 7msec
> Xml = [{ham, [], lists:duplicate(1000, {eggs, [], ["bacon"]})}].
> timer:tc(fun() -> lists:foreach(fun(_) -> xmerl:export_simple(Xml, xmerl_xml) end,
                                  lists:seq(1,1000)) end).
{7043822,ok}

After: 1.3 msec/call
> Xml = [{ham, [], lists:duplicate(1000, {eggs, [], ["bacon"]})}].
> timer:tc(fun() -> lists:foreach(fun(_) -> xmerl:export_simple(Xml, xmerl_xml) end,
                                  lists:seq(1,1000)) end).
{1314278,ok}