"The change to a place of celebration rule has implications beyond spousal leave.... [An] eligible employee would be entitled to FMLA leave to care for the child of a same-sex spouse regardless of whether the employee satisfies the in loco parentis requirement of providing day-to-day care or financial support for the child. An eligible employee would also be entitled to leave to care for a parent's same-sex spouse, whether or not the stepparent ever stood in loco parentis to the employee."