slacktest.postMessageHandler does not preserve blocks

[bendb-instacart]

I'd like to use the slacktest server to verify that I'm sending blocks as expected, but noticed that all messages returned from server.GetSeenOutboundMessages() have "blocks: null.

I looked in to why that might be. As it happens, the postMessageHandler in slacktest/handlers.go ignores blocks entirely. I briefly attempted to add support here but got stuck trying to figure out how to json.Unmarshal(blockBytes, &listOfBlocks) - Block being an interface, and myself being rather new to Go, I'm not sure how to make that work.

Any advice on how to get this working?