Add unit tests for FacebookCanvas::Middleware
user
```ruby
require 'test_helper'

class FacebookCanvas::MiddlewareTest < ActiveSupport::TestCase
  let(:middleware) { FacebookCanvas::Middleware.new(app, request_host, custom_filter) }
  let(:request_host) { %r{.*} }
  let(:custom_filter) { proc { |_env| true } }
  let(:app) { proc { |env| [200, {}, env] } }
  let(:env) {
    {
      "SERVER_NAME" => "test.host",
      "REQUEST_METHOD" => original_request_method,
    }
  }
  let(:original_request_method) { "POST" }

  describe "request_host" do
    describe "matching any host" do
      let(:request_host) { %r{.*} }

      test "modifies request method to GET" do
        env["SERVER_NAME"] = "anything"
        assert_request_method "GET"
      end
    end

    describe "matching speficic host" do
      let(:request_host) { %r{\.fb\.} }

      test "modifies request method to GET if matched" do
        env["SERVER_NAME"] = "test.fb.host"
        assert_request_method "GET"
      end

      test "does not change request method" do
        env["SERVER_NAME"] = "test.facebook.host"
        refute_request_method_change
      end
    end
  end

  describe "custom_filter" do
    let(:custom_filter) do
      proc { |env| env[:pass_custom_filter] }
    end

    test "modifies request method to GET if matched" do
      env[:pass_custom_filter] = true
      assert_request_method "GET"
    end

    test "prevents change of request method" do
      env[:pass_custom_filter] = false
      refute_request_method_change
    end
  end

  describe "with Rails' POST request" do
    before do
      env["rack.request.form_hash"] = {
        "utf8" => "✓"
      }
    end

    test "does not change request method" do
      refute_request_method_change
    end
  end

  describe "with Rails' XHR requests" do
    before do
      env["HTTP_X_REQUESTED_WITH"] = "XMLHttpRequest"
    end

    test "does not change request method" do
      refute_request_method_change
    end
  end

  private

  def assert_request_method(expected)
    status, headers, body = middleware.call(env)

    assert_instance_of Hash, body
    assert_equal expected, body.fetch("REQUEST_METHOD")
  end

  def refute_request_method_change
    assert_request_method original_request_method
  end
end
```
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