fdo#75339 object comparison by reference, not pointer

Reference<>::operator== ensures that the comparison is done right.
::get may return different raw pointers for the same object,
yielding false positives.

Although the same end effect is reached by
  commit	f8115ce72dad45b82b044d9c8f5c253d0514574d
  author	Stephan Bergmann <sbergman@redhat.com>	2014-06-24 09:40:23 (GMT)
  fdo#75339: Substituting XInterface* eq. for object eq. requires queryInterface

This way is more robust against future "clean-up"s.

Change-Id: I2f3c63ac577152c666554b78b88d3779dabe4d45