_.first and _.last should check length

[jdalton]

Array.prototype[0] = 'boo';
_.first([]) // 'boo'

or

var arr = [];
arr[-1] = 'boo'
_.last(arr); // 'boo'

[tgolen]

This had the unfortunately bad side effect of:

Before:

_([]).first(500);
[]

After:

_([]).first(500)
undefined