pollingfile._PollableWritePipe should check for unicode earlier
|Reported by:||zseil||Owned by:||itamar|
branch-diff, diff-cov, branch-cov, buildbot
pollingfile._PollableWritePipe currently checks for outgoing unicode data in checkWork, its equivalent of FileDescriptor.doWrite. This is too late, the user's code will not be notified of this error, it will just break the _PollingTimer to which the pipe is attached. The check for unicode data should be made in write and writeSequence instead.