fix: Passed array should not be sorted

fix #98

Author: fitiskin

lib/index.js

@@ -55,7 +55,7 @@ function percentile(p, list, fn) {
     throw new Error(graterThanHundredError(p));
   }
 
-  list = list.sort(function (a, b) {
+  list = list.slice().sort(function (a, b) {
     if (fn) {
       a = fn(a);
       b = fn(b);