[IZPACK-1152] - Default value selection of choice fields (radio, combo) broken - set attribute does not work like in RC2

Currently, the default value selection of radio and combo fields does not work any longer like in RC2.
For example, instead of marking a radio choice selected like this:
```
<field type="radio" variable="dbms" summaryKey="key.dbms">
	<spec>
		<choice txt="MSSQL" value="mssql" id="text.dbsystemsettings.mssql" />
		<choice txt="Oracle" value="oracle" id="text.dbsystemsettings.oracle" set="true" />
		<choice txt="SAP Hana" value="hdb" id="text.dbsystemsettings.hana" />
	</spec>
</field>
```
the installer expects the default value to be set as as variable value in the "set" attribute of the <spec> tag.
This breaks existing installers.