newRequest creates an API request with proper headers and serializations. // If v is supplied, the value will be JSON encoded and included as the request body. // If the method is GET, it will be query parameters instead. Ref: https://github.com/grezar/go-circleci/blob/c8fca9c02dd337074caeec2515c8f97635597c57/circleci.go#L115-L165