code: func AddContentTypes(types ...string) Option {
  	return func(c *options) error {
  		dst := make([]string, len(c.contentTypes)+len(types))
  		copy(dst, c.contentTypes)
  		copy(dst[len(c.contentTypes):], types)
  		c.contentTypes = dst
  		return nil
  	}
  }
docs: // AddContentTypes allows to specify specific content types to encode.
  // Adds to previous content types.