default FileMixin#as_json options argument to nil

[AlexWheeler]

Fixes #142

• active_support expects as_json to optionally accept an options hash argument line 93 in https://github.com/rails/rails/blob/master/activesupport/lib/active_support/json/encoding.rb calls as_json without an argument so this breaks when trying to jsonify an attachinary file instance
• removes trailing whitespace