* types: ValidatorSet.Update preserves ProposerPriority
This solves the other issue discovered as part of #2718,
where Accum (now called ProposerPriority) is reset to
0 every time a validator is updated.
* update changelog
* add test
* update comment
* Update types/validator_set_test.go
Co-Authored-By: ebuchman <ethan@coinculture.info>