fix encoding problem

UTF-8 is assumed. If a file is displayed, unknown characters are replaced by a UTF-8 question mark
fix diff view encoding error
Merge branch 'correcting_files_view_utf8_error' into correcting412_download_of_ontologies_does_not_work

Conflicts:
	app/helpers/files_helper.rb
	app/views/files/_files_file.html.haml