bullet: restore behavior of in_contact flag
bullet: restore behavior of in_contact flag

c18abad8c3373aff134dd02d099d476d094b0b69 added a call that resets the m_isInContact flag, so this resets it, for better or worse.  We may want to revisit whether #349 was indeed the right fix to #250.

Fixes #453

chandler14362/panda3d,chandler14362/panda3d,chandler14362/panda3d,chandler14362/panda3d,chandler14362/panda3d,chandler14362/panda3d,chandler14362/panda3d,chandler14362/panda3d,chandler14362/panda3d,chandler14362/panda3d