Will add or update an attribute in the XML
<root>
<node attr="false">value</node>
</root>
<Change type="SetAttribute" xPath="/root/node" attributeName="attrVal" value="true" />
<Change type="SetAttribute" xPath="/root/node" attributeName="attr" value="true" />
<root>
<node attr="false" attrVal="true">value</node>
</root>
<root>
<node attr="true">value</node>
</root>