This is to be consistent with the way the mysql2 adapter times queries

Fixed typo in documentation

Directory name in RUNNING_UNIT_TESTS.rdoc [ci skip]

We always know the parameters in advance, so we don't need to build it
in to the method signature

We always set the join type immediately after construction, just make it
part of the constructor and we can skip that step

Port test from cf1904f6 to avoid future regression

Related issue: #11939, #12084

Updated DirtyModel's @changed_attributes hash to be symbol/string agnostic

Conflicts:
	activemodel/CHANGELOG.md

Fix a typo in the code of Testing guide. [ci skip]

Replace the word «Accepts» by «Accept» in the example of custom request headers.

`intiantiated` => `instantiated` [ci skip]

* master:
  copy-edits the new guide with our maintenance policy [ci skip]