[ci skip] Mention the change in nil handling for serialized attributes
in "Upgrading from Rails 3.2 to Rails 4.0"
[ci skip] Clarify the "Serialized Attributes" section

Mention that the change in nil handling for serialized attributes
affects custom coders.
Fix protect_from_forgery docs
Merge pull request #16282 from davidbalbert/patch-1

Fix protect_from_forgery docs [ci-skip]
No verbose backtrace by db:drop when database does not exist.
[ci skip] Fix documentation for @macro and reflection types

Since `@macro` doesn't exist anymore and these reflections are no
longer AssociationReflections but their own types of reflections
based on macro I updated the documentation to match the changes I
made in #16089 and #16198. An `AssociationReflection` that had a
`@macro` of `:has_many` now is a `HasManyReflection`
Merge pull request #16315 from eileencodes/fix-documentation-that-mentions-macro-instance-var

Fix documentation for `@macro` and reflection types
Fix example code of `EachValidator` [ci skip]

We have to specify the `:title` option to really use the
`TitleValidator` defined above.
Merge pull request #16319 from Domon/fix-each-validator-example

Fix example code of EachValidator [ci skip]
Add where condition so select_one returns the correct record.
Merge pull request #16318 from zuhao/refactor_activerecord_adapter_test

make `adapter_test` run order independently.
Merge pull request #16295 from kenn/no_stacktrace_drop_task

No verbose backtrace by db:drop when database does not exist.
Merge pull request #16297 from calebthompson/extract-iterator-method

Extract iterator method in AR::SchemaDumper
Added nodoc to change_table
Merge pull request #16303 from rajcybage/removing_master

remove empty unused method
docs, clarify attribute query methods on numeric columns. Closes #16246.

[ci skip]
Fix typo and remove code block since present is not a method.
Transactions refactoring

Add a transaction manager per connection, so it can controls the
connection responsibilities.

Delegate transaction methods to transaction_manager
savepoint_name should return nil for non-savepoint transactions

Also add test to assets the savepoint name
Merge pull request #16284 from arthurnn/transactions

Transactions refactoring