Clean up class assignments

[bedantaguru]

make sure primitive types and S3 inheritance is maintained correctly.

• This is done loosely in 0.2.x but need
• to do more robustly in 0.3.x

[bedantaguru]

You need to put the class at the front of the vector. Otherwise it’s no longer technically a data frame.

Originally posted by @hadley in tidyverse/dplyr#5211 (comment)