This is all I'd do, honestly

HttpServletRequestRapper.scala

  private def getPreferredHeader(headerName: String, getFun: String => List[String]): List[String] = {
    case class HeaderValue(headerValue: String) {
      val value = headerValue.split(";").head
      val quality = {
        try {
          val headerParameters: Array[String] = headerValue.split(";").tail
          val qualityParameters: Option[String] = headerParameters.find(param => "q".equalsIgnoreCase(param.split("=").head.trim))
          qualityParameters.map(_.split("=", 2)(1).toDouble).getOrElse(1.0)
        } catch {
          case e: NumberFormatException => throw new QualityFormatException("Quality was an unparseable value", e)
        }
      }
    }

    getFun(headerName) match {
      case Nil => Nil
      case nonEmptyList =>
        nonEmptyList.map(headerValue => HeaderValue(headerValue)) // split the value and parameters, parse the quality
          .groupBy(_.quality) // group by quality
          .maxBy(_._1) // find the highest quality group
          ._2 // get the list of highest quality values
          .map(_.value) // return a list of just the values
    }
  }