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"

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