fix bug 415: wrong return in Rotation2D::operator*=
fix bug 415: wrong return in Rotation2D::operator*=

mjbshaw/Eigen,rotorliu/eigen,rotorliu/eigen,mjbshaw/Eigen,rotorliu/eigen,rotorliu/eigen,madlib/eigen_backup,madlib/eigen_backup,madlib/eigen_backup,madlib/eigen_backup,mjbshaw/Eigen,mjbshaw/Eigen