objects example 2 update by xlink

This shows how relations are handled by engine. In this case we edit field/OPIS with data and referred element default_objects:File with exact reference to #File.42

Method: POST
URL: https://biuro.biall-net.pl/SE/version-git/wfs-data.php/default_db/ with GET params:
```
?SERVICE=WFS
&VERSION=1.0.0
&SRSNAME=EPSG:3003
```

data of request to be sent in POST

<Transaction
  xmlns="http://www.opengis.net/wfs"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xmlns:gml="http://www.opengis.net/gml"
  xmlns:default_objects="https://biuro.biall-net.pl/wfs/default_objects"
  version="1.0.0" service="WFS">
  <Update xmlns="http://www.opengis.net/wfs" typeName="default_objects:TestPerms">
    <Property xmlns="http://www.opengis.net/wfs">
      <Name xmlns="http://www.opengis.net/wfs">OPIS</Name>
      <Value xmlns="http://www.opengis.net/wfs">WFS update AccessGroup xlink 79</Value>
    </Property>
    <Property xmlns="http://www.opengis.net/wfs">
      <Name xmlns="http://www.opengis.net/wfs">AccessGroupRead</Name>
      <Value xmlns="http://www.opengis.net/wfs">
        <default_objects:AccessGroupRead xlink:href="https://biuro.biall-net.pl/wfs/default_objects#AccessGroupRead.79"/>
      </Value>
    </Property>
    <Filter xmlns="http://www.opengis.net/ogc">
      <FeatureId xmlns="http://www.opengis.net/ogc" fid="TestPerms.49" />
    </Filter>
  </Update>
</Transaction>

Note: Such transaction makes system associates p5_objects:TestPerms or default_objects:TestPerms to default_objects:AccessGroupRead