Fix marksAt when pointed inside of a text node
Fix marksAt when pointed inside of a text node

Closes #281

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