mime: add as_str() to ParamName and ParamValue

[Fishrock123]

Previously, there was no way to actually get the string values.

This (or something like it) is blocking using http_types::mime in Surf.

Noticed from Surf: https://github.com/http-rs/surf/blob/be10806bb9731c7a1d0f3d36296a1cc61267b775/src/response.rs#L164

[Fishrock123]

The issue I ran into was ultimately solved by .map(|v| v.to_string()).as_deref(), but I feel like this would be better? i.e. map(|v| v.as_str())?

[yoshuawuyts]

💯