Pass a list of dictionaries to the on_failure callback for POST requests

[fblundun]

For GET requests, we pass a list of dictionaries. Each dictionary is of the form

{"p": "ev", ...}

For POST requests we currently pass the whole payload object in string form. Now that we have #91, we should convert this to a list of inner dictionaries for consistency. That way the same callback will work for both GET and POST requests.

[alexanderdean]

Sounds sensible. Behind the scenes, the Snowplow Enrichment process is actually moving to a list of flat dictionaries too for consistency between GET and POST.

[alexanderdean]

This was done, closing