[Rd] 'mean' is not reverted in median() as NEWS says (PR#13731)

[zhengxin at mail.nih.gov]

Full_Name: 
Version: 2.9.0
OS: windows, linux
Submission from: (NULL) (128.231.21.125)


In NEWS, it says "median.default() was altered in 2.8.1 to use sum() rather
	than mean(), although it was still documented to use mean().
	This caused problems for POSIXt objects, for which mean() but
	not sum() makes sense, so the change has been reverted."

But it's not reverted yet.