Improve documentation for Testing Your Mailers [ci skip]
Improve documentation for Testing Your Mailers [ci skip]

The current Basic Test Case example has the following assertion

```
  assert_equal read_fixture('invite').join, email.body.to_s

```

email.body.to_s returns an empty string if both HTML and text templates
exist for a given mailer. This commit adds a note to section 11.2.2
explaining this and also suggests using email.text_part.body.to_s
and email.html_part.body.to_s as alternatives.

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