In 4b685aad the regex in `titlelize` was updated to not match
apostrophes to better reflect the nature of the transformation.
Unfortunately this had the side effect of breaking capitalization
on the first word of a sub-string, e.g:

  >> "This was 'fake news'".titleize
  => "This Was 'fake News'"

This is fixed by extending the look-behind to also check for a
word character on the other side of the apostrophe.

Fixes #28312.