knative - inferred Content-Type from Non-JSON body

If a content-type header is not declared in the action result’s headers, the body is interpreted as `application/json` for non-string values, and `text/html` otherwise. When the `content-type` is defined, the runtime should determine if the response is binary data or plain text and decode the string using a base64 decoder as needed. If the body fail to decode correctly, an error must return to the caller.