```ruby
module Rack
  class MethodOverride
    HTTP_METHODS = %w(GET HEAD PUT POST DELETE OPTIONS PATCH LINK UNLINK)

    METHOD_OVERRIDE_PARAM_KEY = "_method".freeze
    HTTP_METHOD_OVERRIDE_HEADER = "HTTP_X_HTTP_METHOD_OVERRIDE".freeze

    def initialize(app)
      @app = app
    end

    def call(env)
      if allowed_methods.include?(env["REQUEST_METHOD"])
        method = method_override(env)
        if HTTP_METHODS.include?(method)
          env["rack.methodoverride.original_method"] = env["REQUEST_METHOD"]
          env["REQUEST_METHOD"] = method
        end
      end

      @app.call(env)
    end

    def method_override(env)
      req = Request.new(env)
      method = method_override_param(req) ||
        env[HTTP_METHOD_OVERRIDE_HEADER]
      method.to_s.upcase
    end

    private

    def allowed_methods
      ["POST"]
    end

    def method_override_param(req)
      req.POST[METHOD_OVERRIDE_PARAM_KEY]
    end
  end
end
```

Move ["POST"] to a constant
user
```ruby
module Rack
  class MethodOverride
    HTTP_METHODS = %w(GET HEAD PUT POST DELETE OPTIONS PATCH LINK UNLINK)

    METHOD_OVERRIDE_PARAM_KEY = "_method".freeze
    HTTP_METHOD_OVERRIDE_HEADER = "HTTP_X_HTTP_METHOD_OVERRIDE".freeze
    ALLOWED_METHODS = ["POST"]

    def initialize(app)
      @app = app
    end

    def call(env)
      if allowed_methods.include?(env["REQUEST_METHOD"])
        method = method_override(env)
        if HTTP_METHODS.include?(method)
          env["rack.methodoverride.original_method"] = env["REQUEST_METHOD"]
          env["REQUEST_METHOD"] = method
        end
      end

      @app.call(env)
    end

    def method_override(env)
      req = Request.new(env)
      method = method_override_param(req) ||
        env[HTTP_METHOD_OVERRIDE_HEADER]
      method.to_s.upcase
    end

    private

    def allowed_methods
      ALLOWED_METHODS
    end

    def method_override_param(req)
      req.POST[METHOD_OVERRIDE_PARAM_KEY]
    end
  end
end
```
assistant