roxygenize with roxygen2 4.0.2; note @S3method has been deprecated
Merge pull request #29 from yihui/roxygenize

roxygenize with roxygen2 4.0.2; note @S3method has been deprecated
import jsonlite instead of RJSONIO
to_json() is jsonlite::toJSON(dataframe = 'columns'); to_json(orient = 'records') is jsonlite::toJSON()

it is not straightforward to do to_json(orient = 'values') with jsonlite, but I think we can ask @jeroenooms
not sure what the possible trouble was with the default collapse = '\n'
how about just let package authors decide which toJSON() arguments to use?
explicitly import toJSON() only