Hi,

    - Whwn I edit buffer and I decide that was a mistake, I press
      C-c C-c to end the edit. 
    - Gnus however writes the buffer back whether the buffer was
      modified or not.

    The following patch writes the buffer only if buffer was modified.
    gnus-sum patch's + marks are mostly due to changed indentation,
    the changes are in the the `when' statement.

    jari