exml:to_(pretty_)iolist/1 don't properly escape attribute values

[erszcz]

Parsing is OK:

> {ok, T1} = exml:parse(<<"<el attr=\"''''\"/>">>).
{ok,{xmlel,<<"el">>,[{<<"attr">>,<<"''''">>}],[]}}

But printing the term and parsing it again fails:

> R1 = io_lib:format("~ts", [exml:to_iolist(T1)]).
> exml:parse(erlang:iolist_to_binary(R1)).
{error,{"not well-formed (invalid token)",
        <<"<stream><el attr=''''''/></stream>">>}}

What we get by printing is:

> erlang:iolist_to_binary(R1).
<<"<el attr=''''''/>">>

While what we should get is <<"<el attr='&apos;&apos;&apos;&apos;'/>">>. Just to make sure:

> exml:parse(<<"<el attr='&apos;&apos;&apos;&apos;'/>">>) =:= exml:parse(<<"<el attr=\"''''\"/>">>).
true

The same error applies to " / ".

[erszcz]

Fixed by #10.