Fix testing for data-trix-serialize in Safari
Fix testing for data-trix-serialize in Safari

It inconsistently returns dataset.trixSerialize as undefined

GabiGrin/trix,basecamp/trix,ChenMichael/trix,urossmolnik/trix,GabiGrin/trix,urossmolnik/trix,urossmolnik/trix,ChenMichael/trix,basecamp/trix,basecamp/trix,basecamp/trix,GabiGrin/trix,ChenMichael/trix