protected methods return false unless a second argument is passed to
respond_to?.  This only impacts trunk ruby, but will be included in Ruby
2.0.

  https://bugs.ruby-lang.org/projects/ruby-trunk/repository/revisions/34580
Sprockets 2.2.2

Special :heart: release
backport mtime fix, https://github.com/sstephenson/sprockets/pull/321

Signed-off-by: Joel Nimety <jnimety@continuity.net>