Mailers are always async now, and we already discuss
queueing lower down in the guide.

There's no reason for guides to reference old behaviors. They should
be current as of the versions of Rails that they ship with, and including
older information just clutters thing.

I discussed this change with @fxn and he agrees.

We have three or four different introduction sentences to the guides.
After this commit, we use the same one everywhere.

It's obvious that the guide is based on this version of Rails, and may not be backwards compatible.

As issue #7978, the order in which ActionMailer
sends multipart messages could be unintentionally
overwritten if a block is passed to the mail
method. This changes the mail method such that
:parts_order is always respected, regardless of
whether a block is passed to mail.

This reverts commit f5b9ed4f.

REASON: Translation work can not be done in the docrails repository

Change the default test locations to avoid confusion around the common
testing terms "unit" and "functional".
Add new rake tasks for the new locations, while maintaining backwards
compatibility with the old rake tasks.

New testing locations are as follows:

    app/models -> test/models (was test/units)
    app/helpers -> test/helpers (was test/units/helpers)
    app/controllers -> test/controllers (was test/functional)
    app/mailers -> test/mailers (was test/functional)

[ci skip]

Fixed second assert_match - previous one didn't make any sense (if the first passed, the second passed as well)

It's better to pass strings to assert_match - it converts them to regular expressions, escaping special chars like '.'. It seems email.encoded contains some unexpected line breaks (due to encoding), it's better to assert_match email.body.to_s.

Use proc as default argument to recaculate recipients array every time

for "assert /" warnings.
Removed because if somebody will use this code
they will get warnings!