Add test for Response initialized with obj responding to #each

test/spec_response.rb

@@ -439,6 +439,21 @@ def object_with_each.each
     header['content-length'].must_equal '9'
   end
 
+  it "correctly updates content-length when writing when initialized with object body that responds to #each" do
+    obj = Object.new
+    def obj.each
+      yield 'foo'
+      yield 'bar'
+    end
+    r = Rack::Response.new(obj)
+    r.write('baz')
+    r.write('baz')
+    _, header, body = r.finish
+    str = "".dup; body.each { |part| str << part }
+    str.must_equal "foobarbazbaz"
+    header['content-length'].must_equal '12'
+  end
+
   it "doesn't return invalid responses" do
     r = Rack::Response.new(["foo", "bar"], 204)
     _, header, body = r.finish