Fix: bug when same detector is used for more than one input

When using same detector for more than one input (different clouds),
normals are not reset which causes false results or error if the clouds
sizes don't match.

This was reported in the mailing list http://www.pcl-users.org/Problem-with-Harris-3D-keypoint-detector-in-ICCV-2011-feature-matching-Tutorial-td4033177.html