(Bug 51255) Fix selser bug handling subtrees marked 'modified'

* A subtree marked 'modified' are expected to go through regular
  serialization.  However, this bug reported that parts of the
  subtree went through selser.

* Logic that set/unset the 'inModifiedContent' on the WTS state
  object had a bug -- it unconditionally unset the flag which
  effectively reset the 'modified' marker after the first node
  in the modified subtree got processed.

* Verified with a simplified list test case.

* 7 more selser tests now green.

Change-Id: I1dba13273637b01206967b48d5998dcb3ddd55be