Use symbols instead of strings

ActiveSupport::Concern is used all over Rails
This PR will only create 3 new objects as keys are never recreated and
are not subject to garbage collection.

The strings were being uniquely created and garbage collected. I don't
have any performance numbers but this should be better than all of the
GC.
make default value for `:message` on `AM::Errors` explicit.
Removing deprecated rake tasks.

The `rake test:recent` and `rake test:uncommitted` tasks were
deprecated and are now being removed.
Making the rake file for tests easier to read.

I'm defining a new class which modularizes how the `rake test` tasks are
defined and invoked.
Adding documentation to the Rails initialization guide. [ci skip]
Merge pull request #11124 from wangjohn/removing_deprecated_rake_tasks

Removing deprecated rake tasks for tests
Move delegation error constant to inside Module
Review railties changelog [ci skip]
Removing deprecation message and silencing method.

The deprecation rake task is no longer useful because the deprecated
methods have already been removed.

The method for silencing stderr has been removed since it was only
useful for `rake test:uncommitted` which was deprecated and removed.
Improved grammar and replaced 'dbs' slang with 'databases'
Use activerecord 4.0.0 for reporting bugs using the template[ci skip]
Fixing links to accept_language plugin/middleware
Merge pull request #11137 from WvanLelyveld/master

There was a parenthesis in the guides, which resulted in the one link not working, and a second link not even showing. [ci skip]
Merge pull request #11136 from prathamesh-sonpatki/rails4

Use activerecord 4.0.0 for reporting bugs using the template
Merge pull request #11133 from wangjohn/remove_deprecation_rake_task

Removing deprecation message and silencing method.
Merge pull request #11054 from senny/11048_make_default_value_explicit

make default value for `:message` on `AM::Errors` explicit.
fix grammar, change 'an' to 'a' [ci skip]
Merge pull request #11139 from prathamesh-sonpatki/grammar

fix grammar, change 'an' to 'a' [ci skip]
activerecord/examples/simple.rb use master branch activesupport gem
Merge pull request #11142 from tumayun/use_master_branch_activesupport

activerecord/examples/simple.rb use master branch activesupport gem