closed #4424: fix PhysicsBody::setGravity() doesn't work correctly sometimes.
closed #4424: change name _enable->_enabled, _rotationEnable->_rotationEnabled, _gravityEnable->_gravityEnabled
Merge pull request #5854 from boyu0/iss4424_setgravityenable_fix

closed #4424: fix PhysicsBody::setGravityEnable() doesn't work correctly sometimes