treatment for subclasses which don't have definitions of to_f method.
* numeric.c (Integer#to_f, Fixnum#to_f): move to_f method from Fixnum
to Integer.
* ext/-test-/integer/my_integer.rb: define helper class for testing
to_f method for a subclass of Integer.
* ext/-test-/integer/extconf.rb: ditto.
* ext/-test-/integer/init.c: ditto.
* test/-ext-/integer/test_my_integer.rb: examine to_f method for a
subclass of Integer.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@54179 b2dd03c8-39d4-4d8f-98ff-823fe69b080e