removes the second argument of the AS const_missing hook

Ruby does not pass the nesting to const_missing (unfortunately).
That second argument was there in case that changed, Yehuda
sent a patch to MRI

    http://bugs.ruby-lang.org/issues/2740

but there is not much movement there and Matz told me in
Amsterdam there was no immediate plan to pass the nesting.
So let's go back to implement what happens now, and if
in the future we get the nesting then we will adapt this.

Double-checked this with Mr Katz.

activesupport/lib/active_support/dependencies.rb

@@ -168,16 +168,30 @@ def self.exclude_from(base)
         end
       end
 
-      def const_missing(const_name, nesting = nil)
+      def const_missing(const_name)
         klass_name = name.presence || "Object"
 
-        unless nesting
-          # We'll assume that the nesting of Foo::Bar is ["Foo::Bar", "Foo"]
-          # even though it might not be, such as in the case of
-          # class Foo::Bar; Baz; end
-          nesting = []
-          klass_name.to_s.scan(/::|$/) { nesting.unshift $` }
-        end
+        # Since Ruby does not pass the nesting at the point the unknown
+        # constant triggered the callback we cannot fully emulate constant
+        # name lookup and need to make a trade-off: we are going to assume
+        # that the nesting in the body of Foo::Bar is [Foo::Bar, Foo] even
+        # though it might not be. Counterexamples are
+        #
+        #   class Foo::Bar
+        #     Module.nesting # => [Foo::Bar]
+        #   end
+        #
+        # or
+        #
+        #   module M::N
+        #     module S::T
+        #       Module.nesting # => [S::T, M::N]
+        #     end
+        #   end
+        #
+        # for example.
+        nesting = []
+        klass_name.to_s.scan(/::|$/) { nesting.unshift $` }
 
         # If there are multiple levels of nesting to search under, the top
         # level is the one we want to report as the lookup fail.