Async actions preserve single array argument

[wswoodruff]

Fixes #13
Wrote a check for single arg array as a special case, then wrote a test for it

[devinivy]

Hey @wswoodruff could you do a few things here:

• Update the PR title to briefly describe the change you implemented. You can probably just adapt the title from Async actions turn single array argument into multiple handler arguments #13.
• Assign labels and get the milestone setup for the next minor version.
• Move the code you wrote into a new test that also ensures the handler receives the correct arguments, which is the real thing that we want to fix. (Actually in a perfect world we would have been able to fix the handler arguments without changing the action payload, but for now these things are coupled.)

[devinivy]

Lovely!