How to parse request body from Amazon SNS Notification in rails

I'm setting up an endpoint in my rails 3.0 app to receive pushed notifications from an Amazon SNS service.

The request that is posted by Amazon has a JSON payload, but they set content-type on the request as "text/plain", which results in Rails not parsing out the body.

Example post request from Amazon's docs:

POST / HTTP/1.1
x-amz-sns-message-type: Notification
x-amz-sns-message-id: 22b80b92-fdea-4c2c-8f9d-bdfb0c7bf324
x-amz-sns-topic-arn: arn:aws:sns:us-west-2:123456789012:MyTopic
x-amz-sns-subscription-arn: arn:aws:sns:us-west-2:123456789012:MyTopic:c9135db0-26c4-47ec-8998-413945fb5a96
Content-Length: 773
Content-Type: text/plain; charset=UTF-8
Host: myhost.example.com
Connection: Keep-Alive
User-Agent: Amazon Simple Notification Service Agent

{
  "Type" : "Notification",
  "MessageId" : "22b80b92-fdea-4c2c-8f9d-bdfb0c7bf324",
  "TopicArn" : "arn:aws:sns:us-west-2:123456789012:MyTopic",
  "Subject" : "My First Message",
  "Message" : "Hello world!",
  "Timestamp" : "2012-05-02T00:54:06.655Z",
  "SignatureVersion" : "1",
  "Signature" : "EXAMPLEw6JRNwm1LFQL4ICB0bnXrdB8ClRMTQFGBqwLpGbM78tJ4etTwC5zU7O3tS6tGpey3ejedNdOJ+1fkIp9F2/LmNVKb5aFlYq+9rk9ZiPph5YlLmWsDcyC5T+Sy9/umic5S0UQc2PEtgdpVBahwNOdMW4JPwk0kAJJztnc=",
  "SigningCertURL" : "http://ift.tt/1GKDYIE",
  "UnsubscribeURL" : "http://ift.tt/1IQsxjp"
}

So in my controller I can use the request object and read request.body and parse it out myself, like so:

def receive_notification
  if request.content_type =~ /text\/plain/
    body = request.body.read.force_encoding("UTF-8")
    params.merge(JSON.parse(body))
    request.body.rewind
  end
  # ... go on with rest of controller stuff
end

Anyone got a better way to do this? Can we move it back up the chain so my controller isn't worrying about the request object? Is it a bad idea to write a middleware that runs before the parameter parsing and recognizes that a request is from Amazon (by User-Agent, or those custom headers), and changes the content type to "application/json"? (And how do you do that?)