Description:
------------
When trying to sort using the magic attribute '@geodist' which 
should become available after setting a geo anchor, the client 
fails with an error that the attribute is not set.

Reproduce code:
---------------
<?php

$client = new SphinxClient();
$client->setGeoAnchor('post_code_latitude', 'post_code_longitude', 51.494, -0.209);
$client->setSortMode(SPH_SORT_ATTR_ASC, 'price');

var_dump($client->query('foo'));

$client->setSortMode(SPH_SORT_ATTR_ASC', '@geodist');

var_dump($client->query('foo'));

?>


Expected result:
----------------
Result 1:   

["post_code_longitude"]=> float(51.4900016785)
["post_code_latitude"] => float(-0.209999993443)
["@geodist"] => float(10998531)

Result 2:

Same as Result 1
     


Actual result:
--------------
Result 1:   

["post_code_longitude"]=> float(51.4900016785)
["post_code_latitude"] => float(-0.209999993443)
["@geodist"] => float(10998531)

Result 2:

False, Error: sort-by attribute '@geodist' not found