Gracefully handle POSTs with a single object not in an array

[awendland]

The POST endpoint expects objects to be in an array, however, users may submit a single object that isn't nested in an array. The endpoint should check for this an wrap it in an array if necessary.

For example this should work instead of returning an error:

body={"type":"article","uri":"https:\/\/stackoverflow.com\/questions\/5725430\/http-test-server-accepting-get-post-requests"}
-->
response={"statusCode":400,"payload":{"message":"The request body must be in the form [{\"header1\": \"value1\", \"header2\": \"value2\"}, ...]"}}

[awendland]

Also support clients that can't send top level arrays, e.g. allow {"data": [...]}