Adds a test case when we call _.pluck() to objects passing an undefined property as an argument.

Hi.
I've added a test case to `_.pluck()` function in order to ensure return values from it as the specification.
In my opinion, it is intuitive that `_.pluck(objs, key)` returns an empty array in the case when each element of `objs` doesn't have `key` as its property, but it would be better to avoid changing the behavior.