reports: fix exception when using period option and filtering by association.

Fix an exception in reports like this:

* https://betabooru.donmai.us/reports/posts?search[period]=day&search[uploader][name]=evazion

Caused by the `search` method doing a left join instead of a subquery when filtering by a belongs to association.

test/functional/reports_controller_test.rb

@@ -0,0 +1,37 @@
+require "test_helper"
+
+class ReportsControllerTest < ActionDispatch::IntegrationTest
+  context "The reports controller" do
+    context "show action" do
+      context "posts report" do
+        setup do
+          @post = create(:post)
+        end
+
+        should "work" do
+          get report_path("posts")
+
+          assert_response :success
+        end
+
+        should "work with the period param" do
+          get report_path("posts", search: { period: "month" })
+
+          assert_response :success
+        end
+
+        should "work with the group param" do
+          get report_path("posts", search: { group: "uploader" })
+
+          assert_response :success
+        end
+
+        should "work when filtering by a nested association" do
+          get report_path("posts", search: { uploader: { name: @post.uploader.name }})
+
+          assert_response :success
+        end
+      end
+    end
+  end
+end