Fix for has_many_through counter_cache bug

Counter caches were not being updated properly when replacing
has_many_through relationships.

This commit fixes reported issue #7630 in which counter
caches were not being updated properly when replacing
has_many_through relationships

Removed :if / :unless conditions to fragment cache in favour of *cache_i...

cache_if(condition, option, &block) and cache_unless(condition, option, &block).

In the PR #8371 was introduced  conditional options :if and :unless in
the cache method.

    Example:

      <%= cache @model, if: some_condition(@model) do %>
        ...
      <%end%>

This is a good feature but *cache_if* and and *cache_unless*
are more concise and close to the standard of rails view helpers
(ex: link_to_if and link_to_unless).

    Example:

      <%= cache_if condition, @model do %>
      ...
      <%end%>

Remove all the old url helper methods when clear! is called on the
route set because it's possible that some routes have been removed.

This doesn't actually remove old url helper methods as they are
defined in a different module.

This reverts commit 96bcef94.

Conflicts:
	actionpack/CHANGELOG.md

Related to the HTML route inspector changes:
ae68fc38

I was trying to use those files without Rails and that require was
missing.

Add migration examples to Association Basics

[ci skip]

Output routes in :html format

This is apparently used by the railtie to setup the app helpers paths
correctly between initializers. I'll need to check it further.

Don't include debugger gem if jruby is used since it doesn't work on jruby. [ci skip]

* Avoid calling class_eval when not needed
* Remove helpers_path attr accessor, it's defined as a class attribute a
  few lines later
* Avoid creating extra arrays when finding helpers, use flat_map and sort!
* Remove not required refer variable when redirecting :back

Recognize migrations, in folders containing numbers and 'rb'

Closes #8492

Fixed duplicate test case name

Link to Active Record Basics guide [ci skip]

Shush deprecation warning due to initializing a Migrator with migration paths rather than migrations. Use Migrator.open(paths) instead. Thanks @rubys!

By formatting routes for different media (txt/html) we can apply optimizations based on the format. We can include meta-data in the HTML to allow a rich experience while rendering and viewing the routes. This PR shows route helpers as they are used with the `_path` extension, it also has a javascript toggle on the top to switch to `_url`. This way the developer can see the exact named route helper they can use instead of having to modify a base. 

This is one example of an optimization that could be applied. Eventually we can link out to guides for the different columns to better explain what helper, HTTP Verb, Path, and Controller#action indicate. We could even add a route search box that could allow developers to input a given route and see all of the routes that match it. These are stand alone features and should be delivered separately.

Related to the deprecation of Time.utc_time in favor of Time.utc,
in 48583f8b.

Fix rewinding in ActionDispatch::Request#raw_post

Conflicts:
	activerecord/test/cases/base_test.rb

If env['RAW_POST_DATA'] is nil, #raw_post will attempt to set it to
the result of #body (which will return env['rack.input'] if
env['RAW_POST_DATA'] is nil). #raw_post will then attempt to rewind
the result of another call to #body. Since env['RAW_POST_DATA'] has
already been set, the result of #body is not env['rack.input'] anymore.
This causes env['rack.input'] to never be rewound.

The Time.time_with_datetime_fallback, Time.utc_time and Time.local_time
methods were added to handle the limitations of Ruby's native Time
implementation. Those limitations no longer apply so we are deprecating
them in 4.0 and they will be removed in 4.1.

Both String and Date now respond to in_time_zone so we need to
check if the value is a Time or a DateTime.

Rely on the hub files for Time, Date and DateTime classes and
add the requires for String#to_time and String#in_time_zone.

This issue only happens on master due to internal AR refactorings, so
there is no need for a changelog entry.

The test was backported to 3-2-stable to ensure there won't be any regressions.

Explain how to supersede the new dynamic welcome page

The to_time_in_current_zone method doesn't match the naming of the methods
for converting to ActiveSupport::TimeWithZone on Time and DateTime. Since
DateTime inherits from Date that has led to confusion with some users
using the to_time_in_current_zone method with DateTime instances and having
the time part dropped and the UTC offset lost.

This commit fixes this by deprecating the old method and adding a new
in_time_zone method which matches the naming for DateTime and Time. This
should prevent accidently dropping times and UTC offsets when converting
DateTime instances to ActiveSupport::TimeWithZone.

The old, static welcome page instructed developers to delete the
public/index.html file and set a root route.  The new, dynamic welcome
page should tell developers (a) why they're seeing it when it doesn't
correspond to anything in the viewable app source and (b) that it can
be superseded with a root route.