Add the ability to pass the doctype in parameter for Serializer

I need - with forrest - to get the doctype-system and doctype-public in a parameter like following :
<map:serializer logger="sitemap.serializer.xhtml" mime-type="text/html" name="xhtml" pool-grow="2" pool-max="64" pool-min="2"
        src="org.apache.cocoon.serialization.XMLSerializer">
        <parameter name="doctype-public" value="-//W3C//DTD XHTML 1.0 Transitional//EN" />
        <parameter name="doctype-system" value="http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd" />
        <encoding>UTF-8</encoding>
        <indent>yes</indent>
<omit-xml-declaration>no</omit-xml-declaration>
</map:serializer>

here is a patch which apply to the AbstractTextSerializer.
in case the doctype is also found in the childs, child have priority.

Regards,