[BUGFIX] Fix broken flags argument in SortViewHelper

This commit fixes a bug in the SortViewHelper which prevented
the use of combined Flags like "SORT_FLAG_CASE | SORT_STRING" to properly
achieve caseinsensitive sorting.

- use arrayFromArrayOrTraversableOrCSV to split the sortFlags
- update documentation
- throw exception on unknown sortFlags
- fix sorting for empty string values
Merge pull request #626 from mneuhaus/sort_flags

[BUGFIX] Fix broken flags argument in SortViewHelper