Fixing Alibi Detect Server response cloud event data is json marshalled string

[SachinVarghese]

Albi Detect Server logs the detector response to a configured replyURL as a cloud event. The data in the cloud event at the moment is a stringified json sting, for example if the model response is

{
	"data": {
		"outlier": {
			"is_outlier": [1]
		}
	}
}

then the binary could event data is

 b`"{\"data":{\"outlier":{\"is_outlier\":[1]}}}"`

This is because the reponse dict is converted to a json string twice using json.dumps method. This PR removes the extra string convertion, so that the expected binary cloud event data is

 b`{"data":{"outlier":{"is_outlier":[1]}}}`

Note that the detect server response remains the same and the test is updated only for the replyURL cloud event.

[SachinVarghese]

/test integration

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[axsaucedo]

Nice one - notebook and upgrading.md looks good

[seldondev]

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