Implement ToHeaderValues for more string types

[yoshuawuyts]

Implements ToHeaderValues for more string types. Noticed that passing &format!("{}", "value") as the header value didn't work, but format!("{}", "value").as_str() does. This fixes that. Thanks!

[jbr]

Could these be defined in terms of the ToHeaderValues for &'a str impl to reduce duplicated code? Similarly, the String one probably could also be defined as

impl ToHeaderValues for String {
    type Iter = option::IntoIter<HeaderValue>;

    fn to_header_values(&self) -> crate::Result<Self::Iter> {
        self.as_str().to_header_values()
    }
}

That way if changes were needed to the &str impl, it wouldn't have to be repeated in four places