Description:
------------
When giving multiple key/value pairs in the values argument to 
RRFUpdater::update, it seems only the first pair is actually 
being set.

Reproduce code:
---------------
<?php
$rrdFile = '/tmp/test.rrd';

$creator = new RRDCreator($rrdFile, 1313557200, 3600);
$creator->addDataSource("speed:GAUGE:3600:U:U");
$creator->addDataSource("weight:GAUGE:3600:U:U");
$creator->addArchive("AVERAGE:0.5:1:24");
$creator->save();

$updator = new RRDUpdater($rrdFile);
$updator->update(array('speed' => 42, 'weight' => 18), 1313560800);

rm /tmp/test.rrd; php /tmp/rrd.php; rrdtool fetch /tmp/test.rrd AVERAGE -s 1313557200 -e 1313557200
                          speed              weight

1313560800: 4.2000000000e+01 -nan

Expected result:
----------------
I expect both key/value pairs set (speed to 42 and weight to 
18).

Actual result:
--------------
Only the first key/value pair is set.

rm /tmp/test.rrd; php /tmp/rrd.php; rrdtool fetch 
/tmp/test.rrd AVERAGE -s 1313557200 -e 1313557200
                          speed              weight

1313560800: 4.2000000000e+01 -nan