bugfix, when matching multiple paths with `get`, `post`, ...

This problem was introduced with:

https://github.com/rails/rails/commit/d03aa104e069be4e301efa8cefb90a2a785a7bff
routing shorthand syntax works with multiple paths

Closes #9913.

We need to expand the match shorthand syntax for every path.
Changed the guides to use distinct instead of uniq for associations (see
commit a1bb6c8b which changes the naming). I've also provided an
example of how distinctness when inserting into an association can be
achieved.
Merge pull request #10053 from wangjohn/change_docs_for_associations_distinctness

Changed the guides for associations, warning about race conditions when checking for distinctness
each_with_index  => each as we dont use index
Prefer find_by over dynamic finders in rdoc
Merge pull request #10055 from vipulnsward/remove_each_with_index_in_test

each_with_index  => each as we dont use index
Merge pull request #10054 from rubys/use_find_by_in_rdoc

Prefer find_by over dynamic finders in rdoc
Remove rails.png.

No reason to have a file, we've got data-URIs now! It didn't make sense
to me that you no longer have to remove public/index.html, but you still
need to remove rails.png. This means that you won't have to any more
when making a new Rails app.
adding .gigkeep to default app/assets/images
Merge pull request #10056 from steveklabnik/remove_rails_png

Remove rails.png.
Fix tests from rails.png removal.

I'm giving @jlsuttles credit on this commit because she gave me the idea
and the actual converted image in 952289aabfb.
Fix permanent cookie jar accessor typo
Be consistent when talking about cookies, key -> name
:scissors: spacing after private
Merge pull request #10059 from trevorturk/perm-cookie-jar-fix

Fix permanent cookie jar accessor typo
Rename DummyKeyGenerator -> LegacyKeyGenerator
Merge pull request #10060 from trevorturk/cookies-refactoring

Cookies refactoring
Merge pull request #10061 from trevorturk/dummy-key-generator-rename

Rename DummyKeyGenerator -> LegacyKeyGenerator
.gitkeep -> .keep

Mentioned in a comment in e90c2d029edc by @frodsan.