Define `#respond_to_missing?` instead of `#respond_to?`

Ruby 1.9.2 added support for `#respond_to_missing?`, which allows an
object that defines `#method_missing` to behave more like the object
actually had the methods handled by `#method_missing`.

See also: http://blog.marc-andre.ca/2010/11/15/methodmissing-politely/