Fixed a bug in AR::Base#respond_to?

Before:

  >> ActiveRecord::Base.respond_to?(:find_by_something)
  NoMethodError: undefined method `abstract_class?' for Object:Class

After:

  >> ActiveRecord::Base.respond_to?(:find_by_something)
  => false

activerecord/lib/active_record/dynamic_matchers.rb

@@ -6,8 +6,12 @@ module ActiveRecord
     # then we can remove the indirection.
 
     def respond_to?(name, include_private = false)
-      match = Method.match(self, name)
-      match && match.valid? || super
+      if self == Base
+        super
+      else
+        match = Method.match(self, name)
+        match && match.valid? || super
+      end
     end
 
     private

activerecord/test/cases/finder_respond_to_test.rb

@@ -5,6 +5,11 @@ class FinderRespondToTest < ActiveRecord::TestCase
 
   fixtures :topics
 
+  def test_should_preserve_normal_respond_to_behaviour_on_base
+    assert_respond_to ActiveRecord::Base, :new
+    assert !ActiveRecord::Base.respond_to?(:find_by_something)
+  end
+
   def test_should_preserve_normal_respond_to_behaviour_and_respond_to_newly_added_method
     class << Topic; self; end.send(:define_method, :method_added_for_finder_respond_to_test) { }
     assert_respond_to Topic, :method_added_for_finder_respond_to_test