Correctly init the goStruct within the plugins ValidateChange method

The nodes along a path of the copied model (which is a copy of the
underlying goStruct of the plugin) within the ValidateChange method of a
plugin were not properly initialized.

This means if a JSON string was provided that contained a node that was
not initialized an error was thrown. But since the
`ytypes.IgnoreExtraFields` option was set, the node was just ignored and
no error was thrown.

This change correctly initializes the node at the given path and removes
the `ytypes.IgnoreExtraFields` option.