ruby-on-rails - an_instance_of Mail::Message が true を返さない

Question

Rspec matchers に関するこれらのドキュメントによると、呼び出しan_instance_ofは別の呼び出し方法ですinstance_of?(Class)。コンソールでテストすると、これは正常に機能しますが、私の仕様では失敗します。

コンソールで:

m = Mail.new
=> #<Mail::Message:70144097437100, Multipart: false, Headers: >
m.instance_of?(Mail::Message)
=> true 

失敗：

1) IncomingEmailsController should deliver the email with lead info
     Failure/Error: post :create, to: "me@example.com", text: "Content", html: "HTML Content", from: "email@example.com", subject: "Jimmy Bean"
       <UserMailer (class)> received :forward_email with unexpected arguments
         expected: (#<RSpec::Mocks::ArgumentMatchers::InstanceOf:0x007feff35fd148 @klass=Mail::Message>)
              got: (#<Mail::Message:70334278571020, Multipart: true, Headers: <From: email@example.com>, <To: me@example.com>, <Subject: Jimmy Bean>, <Content-Type: multipart/alternative; boundary=--==_mimepart_50c1ffa9e2ba2_2e1e3ff7f8c35ad49574>>, nil)

incoming_emails_controller_spec.rb

describe IncomingEmailsController do

  it "should deliver the email with lead info" do
    # expect
    UserMailer.should_receive(:forward_email).with(an_instance_of(Mail::Message))
    # when
    post :create, to: "me@example.com", text: "Content", html: "HTML Content", from: "email@example.com", subject: "Jimmy Bean"
  end

end

incoming_emails_controller.rb

def create
    # create a Mail object from the params sent by Sendgrid
    prepare_email(params) #returns @email (instance of Mail::Message)
    UserMailer.forward_email(@email).deliver
end

private

def prepare_email(params)
    email = Mail.new
    email.to = params["to"]
    email.from = params["from"]
    email.subject = params["subject"]

    text_part = Mail::Part.new
    text_part.body = params["text"]

    html_part = Mail::Part.new
    html_part.content_type = 'text/html; charset=UTF-8'
    html_part.body = params["html"]

    email.text_part = text_part
    email.html_part = html_part
    @email = email
  end