The wrapper around heapq.nlargest and heapq.nsmallest is much slower
than it's C version.

$ python2.5 -mtimeit -s 'import random; random.seed(123); n=999999;
x=range(n); random.shuffle(x); import _heapq' '_heapq.nlargest(3, x)'
10 loops, best of 3: 142 msec per loop

$ python2.5 -mtimeit -s 'import random; random.seed(123); n=999999;
x=range(n); random.shuffle(x); import heapq' 'heapq.nlargest(3, x)'
10 loops, best of 3: 685 msec per loop


If the key argument is None, there is no need to use the wrapper. This
patch adds an a check to avoid this.