BUG #4822: xmlattributes encodes '&' twice

The following bug has been logged online:

Bug reference:      4822
Logged by:          Itagaki Takahiro
Email address:      itagaki.takahiro@oss.ntt.co.jp
PostgreSQL version: 8.4dev
Operating system:   Linux, Windows
Description:        xmlattributes encodes '&' twice
Details:

=# SELECT xmlelement(name a, xmlattributes('./qa?a=1&b=2' as href), 'Q&A');
                 xmlelement
--------------------------------------------
 <a href="./qa?a=1&amp;b=2">Q&A</a>
(1 row)

'&' in xmlattributes seems to be encoded twice.
( '&' => '&' => '&amp;' )

The bug only exists in 8.4dev;
PostgreSQL 8.3 correctly encodes '&' only once.