PluginDescriptor doesn't read since value of parameter

This causes missing information for code manipulating plugins "normally" through PluginDescriptor read by Maven core from META-INF/maven/plugin.xml.
For example MPLUGIN-319, which parses code to create PluginDescriptor then generate META-INF/maven/plugin.xml, but gets truncated info when getting back PluginDescriptor as read by Maven core...