Given an array a:

> var a = [3,2,1];

> a
< [3, 2, 1]

When mean is called:

> mathjs.mean(a);
< 2

the sort order of a is preserved:

> a
< [3, 2, 1]

However, when median is called:

> mathjs.median(a);
< 2

the sort order of a is lost. a is mutated, and is now sorted ascending:

> a
< [1, 2, 3]

It this expected behaviour? If not, I can provide a PR with failing test. Thanks!