Add custom dict which allows to overwrite setitem
Use custom dict in to_index to filter invalid strings (fixes #511)

This filters vertical tabs from the index dict which is passed to Solr,
as this isn't done by sunburnt (see tow/sunburnt#73).
Merge pull request #554 from nidico/feature-511-fix-invalid-strings

Fix invalid strings in solr input (fixes #511)