Merge pull request #15321 from tgxworld/only_find_routes_as_heads_for_head_request

Call get_routes_as_head only on HEAD requests.
Merge pull request #12080 from ShayDavidson/fix_datetime_partial_dates

Added partial days support to `DateTime`'s `advance` method.
Silence deprecation warning in test

Missed a case that only occured for mysql/mysql2
Merge pull request #15371 from sgrif/sg-silence-deprecation-warning

Silence deprecation warning in test
give all endpoints a superclass
Test Case - Change method name
Merge pull request #15372 from akshay-vishnoi/test-cases

Test Case - Change method name
constraints class does not need the request class anymore
call `serve` with the request on dispatchers
dispatcher doesn't need `call` anymore
move path_parameter encoding check to the request object
rm reset_parameters because we automatically do it from 9ca4839a
Merge branch 'constraints'

* constraints:
  rm reset_parameters because we automatically do it from 9ca4839a
  move path_parameter encoding check to the request object
  dispatcher doesn't need `call` anymore
  call `serve` with the request on dispatchers
  constraints class does not need the request class anymore
  give all endpoints a superclass
  skip the build business if the stack is empty
  stop hardcoding path_parameters and get it from the request
  we do not need to cache rack_app
  a redirect is not a dispatcher by definition, so eliminate test
  push is_a check up to where the Constraints object is allocated
  pass the request object to the application
  pass a request to `matches?` so we can avoid creating excess requests
  nothing is passed to `rack_app` anymore, so rm the params
  one fewer is_a check
  Constraints#app should never return another Constraints object, so switch to if statement
  eliminate dispatcher is_a checks
  push is_a?(Dispatcher) check in to one place
  Always construct route objects with Constraint objects

Conflicts:
	actionpack/lib/action_controller/metal.rb
PARAMETERS_KEY is only used in the request, so move the constant there
rm dead code
Add test case for clear mappings
Deprecate all *_filter callbacks in favor of *_action callbacks

This is the continuation of the work started at
9d62e04838f01f5589fa50b0baa480d60c815e2c
Merge pull request #15373 from akshay-vishnoi/test-cases

Add test case for clear mappings
Merge pull request #15269 from arunagw/aa-fixes-8930

rake railties:install:migrations respects the order of railties
Add CHANGELOG entry for #15269 [ci skip]