eZ XML installer doesn't save array settings correctly if the setting already exist.

Steps to reproduce

Install the following XML:
<eZXMLImporter data_source="/tmp">
<SetSettings>
<SettingsFile name="test.ini" location="settings/override">
<SettingsBlock name="TestSettings">
<VisibilityList>
<value></value>
<value>private</value>
<value>protected</value>
<value>public</value>
</VisibilityList>
</SettingsBlock>
</SettingsFile>
</SetSettings>
</eZXMLImporter>

It will result in a new file being created:
settings/override/test.ini containing:
[TestSettings]
VisibilityList[]
VisibilityList[]=private
VisibilityList[]=protected
VisibilityList[]=public

Running the installation a second time will modify it as:
[TestSettings]
VisibilityList[]=private
VisibilityList[]=protected
VisibilityList[]=public
VisibilityList[]