fix object not found warning in "%<a-%"

[leeper]

The %<a-% produces an object 'x' not found warning when trying to overwrite an existing active binding, for example:

library("pryr")
a <- 1
b %<a-% (a+1)
a
# [1] 1
b
# [1] 2
b %<a-% (a+2)
# Warning message:
# In rm(x, envir = env) : object 'x' not found
a
# [1] 1
b
# [1] 3

Switching to the list argument in rm solves the problem.

[hadley]

Good catch - thanks! Could you please add a bullet to NEWS?

[leeper]

Done.

[hadley]

Thanks!