Add AC::Parameters#to_unsafe_h

As suggested in #16299([1]), this method should be a new public API for
retrieving unfiltered parameters from `ActionController::Parameters`
object, given that `Parameters#to_hash` will no longer work in Rails
5.0+ as we stop inheriting `Parameters` from `Hash`.

[1]: https://github.com/rails/rails/pull/16299#issuecomment-50220919
New applications should use sass-rails 5.0
Pass through the `prepend` option to `AS::Callback`

I'm not sure what's the use case for this, but apparently it broke some apps.
Since it was not the intended result from #16210 I fixed it to not raise an
exception anymore. However, I didn't add documentation for it because I don't
know if this should be officially supported without knowing how it's meant to
be used.

In general, validations should be side-effect-free (other than adding to the
error message to `@errors`). Order-dependent validations seems like a bad idea.

Fixes #18002
Mention that Action Mailer methods are not deferred [ci skip]

Closes #17965, #17968

[Godfrey Chan, Miklós Fazekas]
Merge pull request #18006 from sikachu/add-params-to_unsafe_h

Add AC::Parameters#to_unsafe_h
redis 3.2.0 doesn't allow tcp protocol anymore

Use redis protocol
[guides] Fix method name inconsistency in a mailer example

[ci skip]
Merge pull request #18018 from yuki24/fix-method-name-inconsistency-in-mailer-example

[guides] Fix method name inconsistency in a mailer example
Pass connection rather than alias_tracker

Because we're only using the `connection` so passing the entire tracker
isn't unnecessary.

Eventually only the `connection` will be passed to `add_constraints`
with later refactoring but curretly that's not possible because of
`construct_tables` method.
Fix handling of positional url helper arguments when format is false

There is no need to subtract one from the path_params size when there is
no format parameter because it is not present in the path_params array.

Fixes #17819.
Merge pull request #18020 from binaryberry/handle-positional-args-with-format-false

Fix handling of positional url helper arguments when format is false
Merge pull request #18019 from eileencodes/pass-connection-rather-than-alias_tracker

Pass connection rather than alias_tracker
ParameterFilter shouldn't try to dup symbol keys
allow URL helpers to work with optional scopes
Merge pull request #18021 from nateberkopec/fix-filter-param-symbol-dup

ParameterFilter shouldn't try to dup symbol keys
Merge pull request #18022 from agrobbin/url-helpers-optional-scopes

Allow URL helpers to work with optional scopes
Re-enable capture'ing non-String values

This has been discussed in #17661 and partially reverts the changes made
in 9de83050d3a4b260d4aeb5d09ec4eb64f913ba64 and 986cac73e3c56b3dfa22fd1464f6913e38d32cc3

The test case added to content_for acts as a regression / acceptance test.
Merge pull request #18024 from carpodaster/fix/actionview/capture-non-strings

Re-enable capture'ing non-String values
new guide: constant autoloading and reloading
Edit pass to section 5.1 of getting started guide [ci skip]