Persist glob when replacing a path

When Rails::Paths::Root's []= is used to replace a path it should persist the previous path's glob. Without passing the glob along we get gnarly bugs when trying to wire up things like engines.

    module FooEngine
      class Engine < ::Rails::Engine
        isolate_namespace FooEngine

        config.paths['config/initializers'] = "lib/foo_engine/initializers"
      end
    end

    ## Example of behaviour before this commit.
    #
    # Before the initializer override:
    >> FooEngine::Engine.config.paths["config/initializers"].glob
    => "**/*.rb"

    # After the initializer override:
    >> FooEngine::Engine.config.paths["config/initializers"].glob
    => nil

    ## Example of behaviour after this commit.
    #
    # Before the initializer override:
    >> FooEngine::Engine.config.paths["config/initializers"].glob
    => "**/*.rb"

    # After the initializer override:
    >> FooEngine::Engine.config.paths["config/initializers"].glob
    => "**/*.rb"

railties/lib/rails/paths.rb

@@ -51,7 +51,8 @@ def initialize(path)
       end
 
       def []=(path, value)
-        add(path, :with => value)
+        glob = self[path] ? self[path].glob : nil
+        add(path, :with => value, :glob => glob)
       end
 
       def add(path, options={})

railties/test/paths_test.rb

@@ -200,6 +200,13 @@ def setup
     assert_equal "*.rb", @root["app"].glob
   end
 
+  test "it should be possible to replace a path and persist the original paths glob" do
+    @root.add "app", :glob => "*.rb"
+    @root["app"] = "app2"
+    assert_equal ["/foo/bar/app2"], @root["app"].paths
+    assert_equal "*.rb", @root["app"].glob
+  end
+
   test "a path can be added to the load path" do
     @root["app"] = "app"
     @root["app"].load_path!