Do not create table_name_prefix method on isolated module if the method already exists.

Signed-off-by: NJosé Valim <jose.valim@gmail.com>

railties/lib/rails/engine.rb

@@ -363,8 +363,10 @@ def isolate_namespace(mod)
               _railtie
             end
 
-            define_method(:table_name_prefix) do
-              "#{name}_"
+            unless mod.respond_to?(:table_name_prefix)
+              define_method(:table_name_prefix) do
+                "#{name}_"
+              end
             end
          end
         end

railties/test/railties/engine_test.rb

@@ -683,5 +683,24 @@ class Engine < ::Rails::Engine
       app_generators = Rails.application.config.generators.options[:rails]
       assert_equal :test_unit    , app_generators[:test_framework]
     end
+
+    test "do not create table_name_prefix method if it already exists" do
+      @plugin.write "lib/bukkits.rb", <<-RUBY
+        module Bukkits
+          def self.table_name_prefix
+            "foo"
+          end
+
+          class Engine < ::Rails::Engine
+            isolate_namespace(Bukkits)
+          end
+        end
+      RUBY
+
+      boot_rails
+      require "#{rails_root}/config/environment"
+
+      assert_equal "foo", Bukkits.table_name_prefix
+    end
   end
 end