Update `titlelize` regex to allow apostrophes
In a previous fix 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, eg:
>> "This was 'fake news'".titleize
=> "This Was 'fake News'"
This change fixes this behaviour by extending the look-behind to check for word character apart from an apostrophe.