Fixed `columns_for_distinct` of postgresql adapter
Added partial days support to `DateTime`'s `advance` method.

You can now add partial days (e.g. 2.5.days) to `DateTime` with the advance method.
This was acheived by mimicing the `advance` implementation in `Time`.
Fixed changelog
Added documentation for the :param option for resourceful routing
Remove unnecessary include for integration tests.
WIP: pass existing joins to construct_join_dependency
Merge pull request #15210 from arthurnn/fix_hbtm_reflection

Fix habtm reflection
Conflicts:
    activerecord/CHANGELOG.md
    activerecord/lib/active_record/counter_cache.rb
    activerecord/lib/active_record/reflection.rb
    activerecord/test/cases/reflection_test.rb
Use .to_s on _reflections lookup
Skip if the association was already loaded.
Improve readability of Explain Queries table in guides [ci skip]
Merge pull request #15303 from JohnKellyFerguson/guides-ar-querying

Improve readability of Explain Queries table in guides
integration tests do not need to load fixtures by hand

I don't know when this changed, checked back up to 3.2 and integration
tests instantiate fixtures just fine.
Update all Rails 4.1.0 references in guides to 4.1.1 [ci skip]
Merge pull request #15304 from JohnKellyFerguson/guides-4.1.1

Update all Rails 4.1.0 references in guides to 4.1.1 [ci skip]
unwrap the constraints object on initialization, eliminate loops

Unwrap Constraints objects.  I don't actually think it's possible
to pass a Constraints object to this constructor, but there were
multiple places that kept testing children of this object.  I
*think* they were just being defensive, but I have no idea.
Always construct route objects with Constraint objects
push is_a?(Dispatcher) check in to one place
eliminate dispatcher is_a checks
Remove unnecessary require of Minitest.

Minitest has already been required when calling Minitest.autorun.
Merge pull request #15305 from tgxworld/remove_unnecessary_require

Remove unnecessary require of Minitest.