Preserve class when adding uneval objects

[wch]

This fixes a bug that is the root cause of rstudio/shiny#1178. When adding an aes() object to a ggplot() object, the uneval class is inadvertently dropped. For example:

# When including aes() in the ggplot() call, class is preserved
p <- ggplot(mtcars, aes(wt, mpg))
class(p$mapping)
# [1] "uneval"


# When adding aes() in the ggplot() call, class is lost
p <- ggplot(mtcars) + aes(wt, mpg)
class(p$mapping)
# [1] "list"

This PR preserves the class of the object.

[codecov-io]

Current coverage is 65.54%

Merging #1624 into master will increase coverage by +<.01%

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  Misses       1912       1912          
  Partials        0          0          

1. File R/scale-type.R (not in diff) was modified. more
   • Misses 0
   • Partials 0
   • Hits +1

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[hadley]

Could you please add a unit test?

[hadley]

And a bullet to NEWS.md please?

[hadley]

Thanks!

[lock]

This old issue has been automatically locked. If you believe you have found a related problem, please file a new issue (with reprex) and link to this issue. https://reprex.tidyverse.org/