Pass content as string to MixedContentParser

FYI -- testing master ran into EAD import errors when Fixnum was
passed to MixedContentParser::parse. Casting the content to string
overcame the issue to allow me to continue testing elsewhere.
Merge pull request #94 from mark-cooper/mixed_content_to_s

Pass content as string to MixedContentParser
small tweak to get selenium to pass while headless
adding agents to accession_importer
fixing issue with nil being sent to #t
adding subjects to import on accession CSV