code: public static String to(String str, NamingStyle namingStyle) {
          NamingStyle ns = namingStyle;
          if (str == null) {
              return null;
          }
          if (ns == null) {
              ns = NamingStyle.CAMEL;
          }
          String formatStr;
          switch (ns) {
          case CAMEL:
              formatStr = toLowerCamel(str);
              break;
          case SNAKE:
              formatStr = toSnake(str);
              break;
          case PASCAL:
              formatStr = toPascal(str);
              break;
          case KEBAB:
              formatStr = toKebab(str);
              break;
          default:
              formatStr = toLowerCamel(str);
              break;
          }
          return formatStr;
      }
docs: 指定具体命名风格来生成相应命名风格的字符串.
  
  @param str
  待转换字符串. 若为null，则返回null.
  @param namingStyle
  命名风格的枚举. 若namingStyle为null，CAMEL
  @return namingStyle命名风格的字符串
  @author zhd
  @since 1.0.0