Passing an object to .series() may lead to unexpected behavior

Just a friendly reminder for devs that pass an object to `series()`.
There are platforms that will execute the functions in seemingly
random order. If you or your testsuite rely on order of execution,
it's better to just use an array of functions.
Merge pull request #391 from kvz/patch-1

Passing an object to .series() may lead to unexpected behavior