Clear filtered request attributes between requests in tests

The request attributes filtered_parameters, filtered_env and filtered_path
are memoized for performance reasons. However this can cause unusual
behavior in tests where there are multiple calls to get, post, etc.

Fixes #13803.

(cherry picked from commit 31616068032beb537768787ff3a206b062eb192e)
Merge pull request #13857 from teoljungberg/replace-file-exists-eh

Replace File.exists? with File.exist?
Merge pull request #13867 from mauricio/bug-13861

Fixing issue with activerecord serialization not being able to dump a record after loading it from YAML - fixes #13861

Conflicts:
	activerecord/CHANGELOG.md
Merge pull request #13868 from mauricio/bug-13788

Correctly send the string given to lock! and reload(:lock) to the lock scope - fixes #13788
Conflicts:
	activerecord/CHANGELOG.md
Merge pull request #13612 from eval/issue-13588

Add CreateMigration action
Conflicts:
	railties/CHANGELOG.md
	railties/lib/rails/generators/migration.rb
Merge pull request #13886 from arthurnn/fix_relation_arel

Fix regression on .select method
Conflicts:
	activerecord/CHANGELOG.md
	activerecord/lib/active_record/connection_adapters/abstract/database_statements.rb
	activerecord/test/cases/adapter_test.rb
Merge pull request #12769 from birkirb/master

Boolean parser blows up on a Fixnum.

Conflicts:
	activesupport/CHANGELOG.md

Conflicts:
	activesupport/CHANGELOG.md
Merge pull request #13912 from mauricio/bug-13907

Fixes issue with parsing whitespace content back from database - fixes #13907
Conflicts:
	activerecord/CHANGELOG.md
	activerecord/test/cases/adapters/postgresql/array_test.rb
Merge pull request #13863 from joshjordan/jsj-dont-throw-out-get-params

Do not discard query parameters on requests that use wrap_parameters

Conflicts:
	actionpack/CHANGELOG.md

Conflicts:
	actionpack/CHANGELOG.md
Merge pull request #13688 from jbaudanza/psql-index-exists

PostgreSQL implementation of SchemaStatements#index_name_exists?

Conflicts:
	activerecord/CHANGELOG.md
Fix tests
Merge pull request #13926 from fredwu/doc_polymorphic_sti_var_name_typo

Fixes the camelCase variable name in the docs [ci skip]
Merge pull request #11770 from timruffles/doc_ajax_xhr

be more specific about csrf token and ajax - not whitelisted outside of jquery-rails [ci skip]
`#to_param` returns `nil` if `to_key` returns `nil`. Closes #11399.

The documentation of `#to_key` (http://api.rubyonrails.org/classes/ActiveModel/Conversion.html#method-i-to_key)
states that it returns `nil` if there are no key attributes. `to_param` needs
to be aware of that fact and return `nil` as well.

Previously it raised the following exception:

```
  1) Error:
ConversionTest#test_to_param_returns_nil_if_to_key_is_nil:
NoMethodError: undefined method `join' for nil:NilClass
    /Users/senny/Projects/rails/activemodel/lib/active_model/conversion.rb:65:in `to_param'
    /Users/senny/Projects/rails/activemodel/test/cases/conversion_test.rb:34:in `block in <class:ConversionTest>'
```

Conflicts:
	activemodel/CHANGELOG.md
Merge pull request #13935 from arthurnn/fix_12566

Make sure transaction state resets after commit
docs, AR already auto-detects primary keys. Closes #13946.

This behavior was introduced since Rails 3.1 (207f266ccaaa9cd04cd2a7513ae5598c4358b510)
but the docs were still out of date.

Conflicts:
	activerecord/test/models/mixed_case_monkey.rb
Rails config for raise on missing translations

Add a config to setup whether raise exception for missing translation or
not.