Fix memory leak in Node#to_s.  Internal libxml buffer was not being freed.
Fix memory leak in Node#to_s.  Internal libxml buffer was not being freed.

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