docs: private Boolean getBooleanCapability(String key) {
      Object o = getRawCapabilities().get(key);
      if (o == null) {
        return null;
      } else if (o instanceof Boolean) {
        return (Boolean) o;
      } else if (o instanceof String
              && ("true".equalsIgnoreCase((String) o)
              || "false".equalsIgnoreCase((String) o))) {
        return Boolean.valueOf((String) o);
      } else {
        throw new ClassCastException(String.format(
            "DesiredCapability %s's value should be boolean: found value %s of type %s",
                key, o.toString(), o.getClass().getName()));
      }
    }