ARROW-5541 [R]: cast from negative int32 to uint32 and uint64 are now safe

[romainfrancois]

Minor test update:

library(arrow, warn.conflicts = FALSE)

a <- array(-(1:10))
a$cast(uint32())
#> Error in Array__cast(self, target_type, options): Invalid: Integer value out of bounds
a$cast(uint64())
#> Error in Array__cast(self, target_type, options): Invalid: Integer value out of bounds

^{Created on 2019-06-10 by the reprex package (v0.3.0.9000)}

[nealrichardson]

LGTM