[BUGFIX] SoftReferenceIndex support for more values in class attribute

The SoftReferenceIndex parses and rebuilds typolink tags, but the
support for more than one value in class attribute is missing, because
the values don't get enclosed with quotes on rebuilding.
This leads to lost classes in typolinks in exports from impexp.

Resolves: #58484
Releases: 6.2, 6.1, 4.5
Change-Id: I12ed3be7f5be36254bcee57fcb24bf2a10f92f46
Reviewed-on: https://review.typo3.org/29852
Reviewed-by: Markus Klein
Tested-by: Markus Klein