Fix Formatting in AR Readme

Don't make free form text appear in code blocks [ci skip]
[ci skip] document type_cast_for_write
[ci skip] document ActionDispatch::HTTP::Headers
copy edits [ci skip]
fix formatting errors [ci skip]
[ci skip] Minor improvements on routing.md.
Merge pull request #15029 from JuanitoFatas/docpatch/routing

[ci skip] Minor improvements on routing.md.
Change 'a' to 'an' [ci skip]
Merge pull request #15030 from mechanicles/a-to-an

Change 'a' to 'an' [ci skip]
add tests for the polymorphic_path method
add a test for classes with options
don't manipulate the testclass, just add helpers to the instance
add tests for path based url_for calls
Use ruby 2.1.2 on travis
Merge pull request #15041 from arthurnn/update_ruby

Use ruby 2.1.2 on travis
[ci skip] doc Http::Headers methods
[ci skip] doc ActiveSupport::TimeWithZone#to_s

Current docs are wrong. Does not accept strftime inputs.
Reverts "Fix bugs with changed attributes tracking when transaction gets rollback"

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
* bab48f0a3d53a08bc23ea0887219e8deb963c3d9
  * b0fa7cf3ff8432cc2eef3682b34763b7f8c93cc8.
* 73fb39b6faa9de593ae75ad4e3b8e065ea0e53af
  * 37c238927fbed059de3f26a90d8923fb377568a5.
  * 8d8d4f1560264cd1c74364d67fa0501f6dd2c4fa

Revert "Merge pull request #13166 from bogdan/transaction-magic"
just call the method and assert the return value

do not test internals
Merge pull request #15042 from arthurnn/revert_dirty_transactions

Reverts dirty track restore on transactions rollback