Allow unscoping of preload and eager_load associations

Adds the ability to unscope preload and eager_load associations.
This is the same functionality as `unscope(:includes)` which
may chose to use eager_loading or preloading based on the
overall query complexity. In cases where the type of association
loading is explicit (eager_load and preload), this allows for
unscoping the explicit association loading.  This is helpful when
taking an existing query and performing an aggregate when
has_many associations have explicitly asked for eager_load or
preload. In the example below, unscoping the two has_many
associations removes the extra preload query and eager_load
join.

        query.eager_load!(:has_many_association1)
        query.preload!(:has_many_association2)
        query.unscope(:eager_load, :preload).group(:id).select(:id)

There are also cases where depending on the aggregation
and select the explict preload fails when the select does not
include the preload association key. This issue led me to
write this fix.

activerecord/CHANGELOG.md

@@ -1,3 +1,15 @@
+*   Allow unscoping of preload and eager_load associations
+
+    Added the ability to unscope preload and eager_load associations just like
+    includes, joins, etc. See ActiveRecord::QueryMethods::VALID_UNSCOPING_VALUES
+    for the full list of supported unscopable scopes.
+
+    ```ruby
+    query.unscope(:eager_load, :preload).group(:id).select(:id)
+    ```
+
+    *David Morehouse*
+
 *   Clear locking column on #dup
 
     This change fixes not to duplicate locking_column like id and timestamps.

activerecord/lib/active_record/relation/query_methods.rb

@@ -569,7 +569,8 @@ module ActiveRecord
 
     VALID_UNSCOPING_VALUES = Set.new([:where, :select, :group, :order, :lock,
                                      :limit, :offset, :joins, :left_outer_joins, :annotate,
-                                     :includes, :from, :readonly, :having, :optimizer_hints])
+                                     :includes, :eager_load, :preload, :from, :readonly,
+                                     :having, :optimizer_hints])
 
     # Removes an unwanted relation that is already defined on a chain of relations.
     # This is useful when passing around chains of relations and would like to

activerecord/test/cases/scoping/default_scoping_test.rb

@@ -402,6 +402,20 @@ class DefaultScopingTest < ActiveRecord::TestCase
     assert_equal expected, received
   end
 
+  def test_unscope_eager_load
+    expected = Developer.all.collect(&:name)
+    received = Developer.eager_load(:projects).select(:id).unscope(:eager_load, :select)
+    assert_equal expected, received.collect(&:name)
+    assert_equal false, received.first.projects.loaded?
+  end
+
+  def test_unscope_preloads
+    expected = Developer.all.collect(&:name)
+    received = Developer.preload(:projects).select(:id).unscope(:preload, :select)
+    assert_equal expected, received.collect(&:name)
+    assert_equal false, received.first.projects.loaded?
+  end
+
   def test_unscope_having
     expected = DeveloperOrderedBySalary.all.collect(&:name)
     received = DeveloperOrderedBySalary.having("name IN ('Jamis', 'David')").unscope(:having).collect(&:name)