Fix assumption in splitBlock command that a textblock isn't top-level
Fix assumption in splitBlock command that a textblock isn't top-level

Closes #394

salzhrani/prosemirror,kiejo/prosemirror,salzhrani/prosemirror,kiejo/prosemirror,tzuhsiulin/prosemirror,salzhrani/prosemirror,tzuhsiulin/prosemirror,kiejo/prosemirror,tzuhsiulin/prosemirror