Prevent data.table errors in check_values()

[danschrage]

check_values() relies on selecting variables using a vector of strings corresponding to names. If data is a data.table, this requires the option "with=FALSE". If we instead coerce data to a data.frame at the start of the function, we avoid this error.

This is a bugfix for issue #35.

Please ensure the following before submitting a PR:

• if suggesting code changes or improvements, open an issue first
• for all but trivial changes (e.g., typo fixes), add your name to DESCRIPTION
• for all but trivial changes (e.g., typo fixes), documentation your change in NEWS.md with a parenthetical reference to the issue number being addressed
• if changing documentation, edit files in /R not /man and run devtools::document() to update documentation
• add code or new test files to /tests for any new functionality or bug fix
• make sure R CMD check runs without error before submitting the PR

[leeper]

Thanks. Looks like the best fix for this is to be more data.table aware rather than doing a class coercion. I'm sending a patch.