content-encoding aware string parsing & encoding

Similar to https://github.com/http-rs/surf/issues/101 and https://github.com/http-rs/surf/pull/108 we should make string parsing and encoding aware of the `Content-Type` header.

cc/ @goto-bus-stop; pinging you here since you contributed the patch to Surf; we're planning to move Tide and Surf both over to `http-types`, so figured it'd make sense to include the work you've done here.