`find_in_batches` now returns an `Enumerator` when called without a block, so that it
can be chained with other `Enumerable` methods.
Fix breakage in XmlMini

- Boolean parsing breaks on non strings (i.e. integer 1|0)
- Symbol parsing breaks on non strings.
- BigDecimal parsing breaks due to missing require.
- Update changelog.
psql implementation of #index_name_exists?
Replace Post with Article in getting started guide [ci skip]

- Used Article model instead of Post as it may confuse with 'post'
  requests
- Tried to wrap the guide to 80 chars lines as much as possible.
- Removed unused image
- Fixes #13764
API guidelines: revises warning about +...+ [ci skip]

Modern RDoc accepts a few more things in +...+. In particular symbols work now.

The current regexp in RDoc is https://github.com/rdoc/rdoc/blob/v4.1.1/lib/rdoc/markup/attribute_manager.rb#L133.
adds a section about booleans in the API guidelines [ci skip]
Customize subsecond digits when encoding DateWithTime

The subsecond fraction digits had been hardcoded to 3. This forced all
timestamps to include the subsecond digits with no way to customize the
value. While the subsecond format is part of the ISO8601 spec, it is not
adhered to by all parsers (notably mobile clients). This adds the
ability to customize the number of digits used, optionally setting them
to 0 in order to eliminate the subsecond fraction entirely:

ActiveSupport::JSON::Encoding.subsecond_fraction_digits = 0
Consolidate JSON encoding tests in one file
Rename subsecond_fraction_digits option to time_precision
Add CHANGELOG entry for #11464
Add support for JSON time_precision to Time and DateTime
Make ActiveSupport::TimeWithZone#xmlschema consistent

Both Time#xmlschema and DateTime#xmlschema can accept nil values for the
fraction_digits parameter. This commit makes this so for TimeWithZone
values as well.
Display value when raising due to unscope() issues

Hopefully make it easier to debug errors. e.g

Before:

    RuntimeError:
       unscope(where: "deleted_at") failed: unscoping String is unimplemented.

After:

    RuntimeError:
       unscope(where: "deleted_at") failed: unscoping String "'t'='t'" is unimplemented.
Ruby on Rails 4.1 Release Notes: Fix spelling [ci skip]
Merge pull request #13849 from fgo/patch-12

Ruby on Rails 4.1 Release Notes: Fix spelling [ci skip]
docs, clarify usage of `action_mailer.default_options`. [ci skip]. Closes #13820.
Ability to specify multiple contexts when defining a validation.

Example:

validates_presence_of :name, on: [:update, :custom_validation_context]
Merge pull request #13848 from huoxito/tell-value

Display value when raising due to unscope() issues
Rails config for raise on missing translations

Add a config to setup whether raise exception for missing translation or
not.
Merge pull request #13754 from vpuzzella/master

Ability to specify multiple contexts when defining a validation.