Consistency in taking character input

[michaellevy]

Some functions seem to take a character input fine, which I assumed was by design. fct_expand at least doesn't, and returns this rather unintuitive error: Error in mutate_impl(.data, dots) : Evaluation error: is.list(x) is not TRUE.

[hadley]

Fixed - in the future, feel free to submit a PR yourself if you spot a function that is missing check_factor().