Allow methods arity below -1 in assert_responds.

Every method from MRI's core classes is written in C. This means
Method#arity always returns -1 for methods with a variable number of
arguments. This is not the case with Rubinius, where, for example
Array#slice! is implemented in Ruby and has arity -2, since is
defined as def slice!(start, length = undefined)

activerecord/test/cases/relation/delegation_test.rb

@@ -9,10 +9,11 @@ module ActiveRecord
     def assert_responds(target, method)
       assert target.respond_to?(method)
       assert_nothing_raised do
-        case target.to_a.method(method).arity
-        when 0
+        method_arity = target.to_a.method(method).arity
+
+        if method_arity.zero?
           target.send(method)
-        when -1
+        elsif method_arity < 0
           if method == :shuffle!
             target.send(method)
           else