Add check for a node's document when calling Document#root=
Add check for a node's document when calling Document#root=

sferik/libxml-ruby,sferik/libxml-ruby,increments/libxml-ruby,sferik/libxml-ruby,xml4r/libxml-ruby,xml4r/libxml-ruby,increments/libxml-ruby,xml4r/libxml-ruby,sferik/libxml-ruby,sferik/libxml-ruby,sferik/libxml-ruby,increments/libxml-ruby