Email does not support relative links since there is no implicit host. Therefore all links inside of emails must be fully qualified URLs. All path helpers are now deprecated. When removed, the error will give early indication to developers to use `*_url` methods instead.

Currently if a developer uses a `*_path` helper, their tests and `mail_view` will not catch the mistake. The only way to see the error is by sending emails in production. Preventing sending out emails with non-working path's is the desired end goal of this PR.

Currently path helpers are mixed-in to controllers (the ActionMailer::Base acts as a controller). All `*_url` and `*_path` helpers are made available through the same module. This PR separates this behavior into two modules so we can extend the `*_path` methods to add a Deprecation to them. Once deprecated we can use this same area to raise a NoMethodError and add an informative message directing the developer to use `*_url` instead.

The module with warnings is only mixed in when a controller returns false from the newly added `supports_relative_path?`.

Paired @sgrif & @schneems

The display name in the email "to" field needs to be quoted otherwise
your email-sending service will fail when trying to deliver mail to
user's with commas in their name (i.e. John Smith, M.D.).

* Wrap 80 words if possible.
* Make attachment hash options more readable. (L239-L243)

  Before:

  ![screenshot 2014-06-11 02 59 54](https://cloud.githubusercontent.com/assets/1000669/3235432/a91d0464-f0d2-11e3-9e4a-6af2b2a83f1a.png)

   After:

  ![screenshot 2014-06-11 02 59 36](https://cloud.githubusercontent.com/assets/1000669/3235428/a47b28dc-f0d2-11e3-806c-cfaedd3f15df.png)

* L619 Wrap "location" in code: `"location"`.
* L628 Use actual section name.
* L697 Remove unnecessary `./` when refer to other guide.

[ci skip]

Minor align BR tags with HTML void - with reference to the generators code change - https://github.com/rails/rails/commit/f038d4cc5fe1ac21f92c7e32c61ff85a0c816f3f

* Indent after private/protected
* Ruby >= 1.9 syntax for hashes
* Prefer method { do_stuff } instead of method{do_stuff} for single-line blocks.

[ci skip]

This reverts commit 70d6e16f, reversing
changes made to ea4db3bc.

Seems to be a code merge done by mistake.

Minor align BR tags with HTML void - with reference to the generators code change - https://github.com/rails/rails/commit/f038d4cc5fe1ac21f92c7e32c61ff85a0c816f3f

Attachments were previously covered so remove the useless part. Just
move a note about multipart headers set when the mail method is
triggered to the kept section.

Closes #9325.

I adjusted the example and the description in the testing guide and
simply linked from the mailer to the testing guide. This way we don't
have to maintain two separate places.

Removed template root from actionmailer configuration options as it seems to have been pulled out completely in favor of explicit definition via template_path / template_name.

thanks @danielpuglisi

[ci skip]

Introduce ./bin for your app's executables: bin/bundle, bin/rails, bin/rake. Executable scripts are versioned code like the rest of your app. To generate a stub for a bundled gem: 'bundle binstubs unicorn' and 'git add bin/unicorn'

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.