func (h *HTTPBodyMarshaler) Marshal(v interface{}) ([]byte, error) {
	if httpBody, ok := v.(*httpbody.HttpBody); ok {
		return httpBody.Data, nil
	}
	return h.Marshaler.Marshal(v)
}
Marshal marshals "v" by returning the body bytes if v is a
google.api.HttpBody message, otherwise it falls back to the default Marshaler.
package runtime
import (
	"google.golang.org/genproto/googleapis/api/httpbody"
)
func SetHTTPBodyMarshaler(serveMux *ServeMux) {
	serveMux.marshalers.mimeMap[MIMEWildcard] = &HTTPBodyMarshaler{
		Marshaler: &JSONPb{OrigName: true},
	}
}
type HTTPBodyMarshaler struct {
	Marshaler
}
func (h *HTTPBodyMarshaler) ContentType() string {
	return h.ContentTypeFromMessage(nil)
}
func (h *HTTPBodyMarshaler) ContentTypeFromMessage(v interface{}) string {
	if httpBody, ok := v.(*httpbody.HttpBody); ok {
		return httpBody.GetContentType()
	}
	return h.Marshaler.ContentType()
}