Fix bug in TextProperty DOM mapping.
Fix bug in TextProperty DOM mapping.

TypesetIO/substance,substance/substance,TypesetIO/substance,jacwright/substance,abhaga/substance,andene/substance,podviaznikov/substance,stencila/substance,andene/substance,jacwright/substance,substance/substance,michael/substance-1,michael/substance-1,abhaga/substance,podviaznikov/substance