indexer: fix for term washing
- FIX Avoids an exception from happening when passing a unicode string to the BibIndex engine washer. (closes #2981)
- NOTE The lower_index_term() now returns the term as a Unicode string which can have an impact on custom tokenizers and regular indexing.
- Adds a unit test to cover this case.
Co-authored-by: Eamonn Maguire <eamonn.maguire@cern.ch>
Signed-off-by: Jan Aage Lavik <jan.age.lavik@cern.ch>