Adding hash post array

Okay, sorry to break these into so many pull request but I wanted you to be able to see them as different features in case you didn't agree with one of them. This one adds a helper to Hash so that users can convert to POST arrays. I came across this when I was trying to call the RedcapAPI and didn't know the format to send an array. I added the example in the README for the case I was trying to solve.