Fix and modernize specs

No change to functionality or test coverage, but specs now PASS and
do not give any deprecation warnings.
Support second argument to #respond_to?

This matches Object's interface in Ruby 1.8.7--2.1.1 (at least), and avoids a
warning that our #respond_to? "is old fashion which takes only one parameter".
Merge pull request #10 from philipmw/fix-and-modernize-specs

Fix and modernize specs
Merge pull request #11 from philipmw/respond-to-second-argument

Support second argument to #respond_to?