Fix build failures with `--features "unstable"`
Fix build failures with `--features "unstable"`

With the unstable feature there's a specializable impl of
`FromSqlRow<ST, DB> for Option<T>`, so the tuple impl isn't able to
resolve what the value of `ST` is just from `Self`. Type annotations fix
the issue.

sgrif/diesel,sgrif/diesel,diesel-rs/diesel,diesel-rs/diesel