Fix broken proc syntax for 1.9.3

This lets us avoid

1. A slow call to Hash#slice
2. An is_a? test
3. Extra hash allocations (from slice)
4. String allocations

etc.

avoids extra hash allocations on each call

this is due to:

  https://bugs.ruby-lang.org/issues/5994

Documentation, add examples of using an Enum scope [ci skip]

When scanning the Enum API page I found it easy to miss that scopes are generated.  Adding an example clarifies use and has more... *pop*...

Only need MINOR version level to test Ruby 2.1.x on Travis

cc http://blog.travis-ci.com/2014-04-28-upcoming-build-environment-updates/

Reverts dirty track restore on transactions rollback

do not test internals

We are reverting these commits, because there are other caveats related
to dirty attributes not being restored when a transaction is rollback.
For instance, nested transactions cannot proper restore the dirty
attributes state after a rollback.

At the moment, the dirty attributes are scoped by the transaction.
When we call `.save` on a record, the dirty attributes will be reset even
if the transaction gets rollback.

[related #13166]
[related #15018]
[related #15016]
[closes #15019]

This reverts commits
* bab48f0a
  * b0fa7cf3.
* 73fb39b6
  * 37c23892.
  * 8d8d4f15

Revert "Merge pull request #13166 from bogdan/transaction-magic"

Use ruby 2.1.2 on travis

Change 'a' to 'an' [ci skip]

[ci skip] Minor improvements on routing.md.

I wanted the middle model to be anonymous, but we have to give it a name
for marshal support. :'(

This test was inspired by #14866. That PR would break this
functionality and we should make sure we notice that in the future.

Habtm relation size calculation fix. Fixes #14913, Fixes #14914.

This resulted in some unrelated errors on Traivs:

  1) Error:
PostgresqlUUIDTest#test_treat_blank_uuid_as_nil:
ActiveRecord::StatementInvalid: PG::UndefinedColumn: ERROR:  column "thingy" of relation "uuid_data_type" does not exist
LINE 1: INSERT INTO "uuid_data_type" ("thingy") VALUES ($1) RETURNIN...
                                      ^
: INSERT INTO "uuid_data_type" ("thingy") VALUES ($1) RETURNING "id"

/cc @zuhao

HABTM should fall back to using the normal CollectionAssociation's size calculation if the collection is not cached or loaded.

This addresses both #14913 and #14914 for master.

Mismatch branch name between text and code example [ci skip]

[ci skip]

Dup the changed_attributes otherwise we could lose them

Use Rails::Paths::Path#existent in database_configuration

Database configuration was trying to load the first path that
config.paths['config/database'] was returning even if the path didn't
exist in the filesystem.

Because Rails::Paths::Path has the possibility to return more than 1
path (as an array), database_configuration should filter down the paths
to the existing one and then load the first one.

This would make it possible to move the database.yml file and add the
new path to paths['config/database'] and still load the configurations.

Closes #14989

Fix Hash#deep_merge bug and improve documentation — resolves #12738