Support Other JSON Content-Types in content_type.go

[Evynglais]

Hi,

At the moment, isJSON(contentType string) bool in content_type.go only allows a specific subset of JSON Content-Types*. This means (for example), we are unable to use application/protobuf+json unless we escape the data and pass it as a single string element.

*ApplicationJSON, TextJSON, ApplicationCloudEventsJSON, ApplicationCloudEventsBatchJSON

This list of types should be expanded (or preferably, the method refactored to avoid hard-coding a list of strings) so a more generic set of JSON Content-Types can be supported. Alternatively, a way to register our own Content-Type so that it can be treated as a JSON object rather than a string.

Thanks,
Evynglai

[Evynglais]

Examples

Structured Single

"Content-Type": "application/cloudevents+json"
{
	"specversion": "1.0",
	"id": "event",
	"source": "test/source",
	"type": "test.type",
	"datacontenttype": "application/protobuf+json",
	"data": { "identifier": "event-1" }
}

Result:

failed to parse CloudEvents HTTP response into single event: unexpected data payload for media type "application/protobuf+json", expected a string: ReadString: expects " or n, but found {, error found in #10 byte of ...|	"data": { "identifi|..., bigger context ...|tenttype": "application/protobuf+json",
        	            		"data": { "identifier": "batch-event-1" }
        	            	}|...

Structured Batch

"Content-Type": "application/cloudevents-batch+json"
[{
	"specversion": "1.0",
	"id": "batch-event-1",
	"source": "test/source",
	"type": "test.type",
	"datacontenttype": "application/protobuf+json",
	"data": { "identifier": "batch-event-1" }
},
{
	"specversion": "1.0",
	"id": "batch-event-2",
	"source": "test/source",
	"type": "test.type",
	"datacontenttype": "application/protobuf+json",
	"data": { "identifier": "batch-event-2" }
}]

Result:

failed to parse CloudEvents HTTP response into batched events: unexpected data payload for media type "application/protobuf+json", expected a string: ReadString: expects " or n, but found {, error found in #10 byte of ...|	"data": { "identifi|..., bigger context ...|tenttype": "application/protobuf+json",
        	            		"data": { "identifier": "batch-event-1" }
        	            	}|...