How to test email headers using RSpec - ruby-on-rails

Question

I'm using SendGrid's SMTP API in my Rails application to send out emails. However, I'm running into troubles testing the email header ("X-SMTPAPI") using RSpec.
Here's what the email looks like (retrieving from ActionMailer::Base.deliveries):
#<Mail::Message:2189335760, Multipart: false, Headers:
<Date: Tue, 20 Dec 2011 16:14:25 +0800>,
<From: "Acme Inc" <contact#acmeinc.com>>,
<To: doesntmatter#nowhere.com>,
<Message-ID: <4ef043e1b9490_4e4800eb1b095f1#Macbook.local.mail>>,
<Subject: Your Acme order>, <Mime-Version: 1.0>,
<Content-Type: text/plain>, <Content-Transfer-Encoding: 7bit>,
<X-SMTPAPI: {"sub":{"|last_name|":[Foo],"|first_name|":[Bar]},"to":["foo#bar.com"]}>>
Here's my spec code (which failed):
ActionMailer::Base.deliveries.last.to.should include("foo#bar.com")
I've also tried various method to retrieve the header("X-SMTPAPI") and didn't work either:
mail = ActionMailer::Base.deliveries.last
mail.headers("X-SMTPAPI") #NoMethodError: undefined method `each_pair' for "X-SMTPAPI":String
Help?
Update (answer)
Turns out, I can do this to retrieve the value of the email header:
mail.header['X-SMTPAPI'].value
However, the returned value is in JSON format. Then, all I need to do is to decode it:
sendgrid_header = ActiveSupport::JSON.decode(mail.header['X-SMTPAPI'].value)
which returns a hash, where I can do this:
sendgrid_header["to"]
to retrieve the array of email addresses.

Answer 1

The email_spec gem has a bunch of matchers that make this easier, you can do stuff like
mail.should have_header('X-SMTPAPI', some_value)
mail.should deliver_to('foo#bar.com')
And perusing the source to that gem should point you in the right direction if you don't want to use it e.g.
mail.to.addrs
returns you the email addresses (as opposed to stuff like 'Bob ')
and
mail.header['foo']
gets you the field for the foo header (depending on what you're checking you may want to call to_s on it to get the actual field value)

Answer 2

Repeating some of the other advice here, in more modern rspec syntax:
RSpec.describe ImportFile::Mailer do
describe '.file_error' do
let(:mail) { described_class.file_error('daily.csv', 'missing header') }
it { expect(mail.subject).to eq("Import error: missing header in daily.csv") }
it { expect(mail.header['X-source-file'].to_s).to eq ('daily.csv') }
end
end