Fix checking for successful steps in history
Fix checking for successful steps in history

Closes #280

kiejo/prosemirror,adrianheine/prosemirror,kiejo/prosemirror,salzhrani/prosemirror,adrianheine/prosemirror,mattberkowitz/prosemirror,tzuhsiulin/prosemirror,kiejo/prosemirror,tzuhsiulin/prosemirror,salzhrani/prosemirror,adrianheine/prosemirror,salzhrani/prosemirror,tzuhsiulin/prosemirror,mattberkowitz/prosemirror