Algorithm change: Camel case string to underscore string supports abbreviation

It's a possible breaking change, but quite unlikely.

New algorithm can detect "abbreviation" in a string. For example, in string `HTTPServer`, `HTTP` is an abbreviation and will be converted to `http` instead of `h_t_t_p`.

Following are typical cases
- `HTTPServer` => `http_server`
- `ALL` => `all`
- `NoHTTPS` => `no_https`
