fill_na() --> fill_gaps()

[earowang]

Three functions handle implicit missing values (time gaps): has_gaps(), count_gaps() and fill_na().

fill_na() somehow can be interpreted as "filling na with values" like tidyr::replace_na(). But this is not what fill_na() does. It attempts to fill default NA to these time gaps. Would fill_gaps() be a better name and consistent with other two functions?

@dicook @robjhyndman @mitchelloharawild