Ruby: Fix rack response tracking

Use type tracking instead of getReturningNode, which seems to be faster
and works correctly for the cases I've tried.

ruby/ql/test/library-tests/frameworks/rack/Rack.expected

@@ -1,3 +1,4 @@
 | rack.rb:1:1:5:3 | HelloWorld | rack.rb:2:12:2:14 | env |
 | rack.rb:7:1:16:3 | Proxy | rack.rb:12:12:12:18 | the_env |
 | rack.rb:18:1:31:3 | Logger | rack.rb:24:12:24:14 | env |
+| rack.rb:45:1:61:3 | Baz | rack.rb:46:12:46:14 | env |

ruby/ql/test/library-tests/frameworks/rack/rack.rb

@@ -41,3 +41,21 @@ class Bar
     nil
   end
 end
+
+class Baz
+  def call(env)
+    run(env)
+  end
+
+  def run(env)
+    if env[:foo] == "foo"
+      [200, {}, "foo"]
+    else
+      error
+    end
+  end
+
+  def error
+    [400, {}, "nope"]
+  end
+end