Ported instance_exec from rspec-expecations.

instance_exec is undefined on ruby 1.8.6.  Since it's used in lib/rspec/core/example_group.rb:179, we need to define it here as well, so that rspec-core can be used on ruby 1.8.6 without rspec-expectations.

lib/rspec/core/object_extensions.rb

@@ -8,6 +8,32 @@ def describe(*args, &example_group_block)
       end
 
       alias :context :describe
+
+      unless respond_to?(:instance_exec)
+        # based on Bounded Spec InstanceExec (Mauricio Fernandez)
+        # http://eigenclass.org/hiki/bounded+space+instance_exec
+        # - uses singleton_class of matcher instead of global
+        #   InstanceExecHelper module
+        # - this keeps it scoped to this class only, which is the
+        #   only place we need it
+        # - only necessary for ruby 1.8.6
+        def instance_exec(*args, &block)
+          singleton_class = (class << self; self; end)
+          begin
+            orig_critical, Thread.critical = Thread.critical, true
+            n = 0
+            n += 1 while respond_to?(method_name="__instance_exec#{n}")
+            singleton_class.module_eval{ define_method(method_name, &block) }
+          ensure
+            Thread.critical = orig_critical
+          end
+          begin
+            return send(method_name, *args)
+          ensure
+            singleton_class.module_eval{ remove_method(method_name) } rescue nil
+          end
+        end
+      end
     end
   end
 end