[Bug]: `c` method for teal_data doesn't concatenate `@join_keys`

[gogonzo]

What happened?

d1 <- teal_data(iris = iris, join_keys = join_keys(join_key("iris", "iris", "iris_key"))
d2 <- teal_data(mtcars = mtcars, join_keys = join_keys(join_key("mtcars", "mtcars", "mtcars_key"))

c(d1, d2)@join_keys
# <only iris_key>