Fix reset_session in Rails 4 (TORQUE-1183)

Prior to Rails 4, Rails took care of clearing the contents of the
session hash on our behalf after session.destroy was called. In Rails
4, this was changed to only call session.destroy so we'll clear the
contents ourselves and invalidate the Java session.

Without this change, under Rails 4 the user would get a new Java
session but it contain the exact same session data as their previous
session, effectively making reset_session do nothing.