Unwrap Constraints objects.  I don't actually think it's possible
to pass a Constraints object to this constructor, but there were
multiple places that kept testing children of this object.  I
*think* they were just being defensive, but I have no idea.

Update all Rails 4.1.0 references in guides to 4.1.1 [ci skip]

I don't know when this changed, checked back up to 3.2 and integration
tests instantiate fixtures just fine.

Improve readability of Explain Queries table in guides

rake stats for engines

documentation link fix and removing unused reader

fixed capitalization [ci skip]

avoid using alias in generators

[Guides] Add documentation for using Rails with a reverse proxy [ci skip]

Fixes URL generation with trailing_slash: true

Conflicts:
	actionpack/lib/action_dispatch/http/url.rb

This mirrors the layout of abstract adapter and puts the definitions
inside the `PostgreSQL` namespace (no longer under the adapter namespace).

/cc @kares

This reverts commit 7466d6a9, reversing
changes made to b850c4c5.

This was an accidental merge. A new PR will follow.

/cc @schneems

print generators on failed generate

[ci skip] Improve doc for ModuleConstMissing.guess_for_anonymous

Let's say we just ran:

```
$ rails g migrate add_click_to_issue_assignment
```

We will get an error that looks like: 

```
Could not find generator migrate.
```

This patch adds all existing migrations to the output to make it easier for a developer to find a valid migration. 

```
Could not find generator "migrate". Please select a valid generator:
Rails:
  assets
  controller
  generator
  helper
  integration_test
  mailer
  migration
  model
  resource
  scaffold
  scaffold_controller
  task
```


It would be nice to do some spelling detection and suggest alternatives, but for now this should  help.

Remove special case in schema dumper for decimal without scale

Fix router visualizer CSS and JS resources

I know, it's crazy.

Future port c8ddb617 [ci skip]

this also changes the constructor.  We don't need to pass more options
than "defaults" (whatever defaults are, ugh. probably another hash of
stupid stuff).

Remove duplicated parameter check on #where!

It seems that #where! is not designed to be used as a chained where.
See initial implementation at 8c2c6051.
So, no need to check twice.

We should not test #where!

https://github.com/rails/rails/pull/15285#discussion_r13018316

update select_tag docs for customizing id

Simplify the code in schema cache