Might want to do input validation for ;

When building a cookie like this:

```
cookie::Cookie::build("name", "somevalue; Max-Age=86400")
```

This gets injected into the cookie. Especially when taking user input as a cookie value, that might result in undesirable behavior. There might be more characters than ; to check for.

I'm not sure if this crate is the right place to do validation. If not, feel free to close.