Add `Relation#load`

This method explicitly loads the records and then returns `self`.

Rather than deciding between "do I want an array or a relation?",
most people are actually asking themselves "do I want to eager load
or lazy load?" Therefore, this method provides a way to explicitly
eager-load without having to switch from a `Relation` to an array.

Example:

    @posts = Post.where(published: true).load

1 files changed, 8 insertions, 0 deletions

diff --git a/activerecord/test/cases/relations_test.rb b/activerecord/test/cases/relations_test.rb
index 4aa1977cf9..1fbd76bc03 100644
--- a/activerecord/test/cases/relations_test.rb
+++ b/activerecord/test/cases/relations_test.rb
@@ -1342,4 +1342,12 @@ class RelationTest < ActiveRecord::TestCase
     node = relation.arel.constraints.first.grep(Arel::Attributes::Attribute).first
     assert_equal table_alias, node.relation
   end
+
+  test '#load' do
+    relation = Post.all
+    assert_queries(1) do
+      assert_equal relation, relation.load
+    end
+    assert_no_queries { relation.to_a }
+  end
 end