Direct call to S3 method from bit64

Here is a direct call to an S3 method https://github.com/ropensci/writexl/blob/7c06c338cdddc0cc1c078afe3173a71a6efe33ff/R/write_xlsx.R#L65 This will stop working in the future when this symbol is un-exported. Prefer calling the generic directly: `as.double()`, and letting S3 dispatch do its thing. See https://github.com/r-lib/bit64/issues/76