Fix EE load balancing crashing on Rails 5

Rails 5 added the `preparable` argument to `select_all`, which was added
to help prepared statement caching via
https://github.com/rails/rails/commit/2a56b2d90d4fed8548e3a1e7a7b206454858c872.

This argument is not needed by the load balancer since prepared
statements are disabled, but it needs to be present or a method
signature error is thrown.

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/8692

ee/lib/gitlab/database/load_balancing/connection_proxy.rb

@@ -30,7 +30,7 @@ module Gitlab
           read_using_load_balancer(:select, args)
         end
 
-        def select_all(arel, name = nil, binds = [])
+        def select_all(arel, name = nil, binds = [], preparable: nil)
           if arel.respond_to?(:locked) && arel.locked
             # SELECT ... FOR UPDATE queries should be sent to the primary.
             write_using_load_balancer(:select_all, [arel, name, binds],

ee/spec/lib/gitlab/database/load_balancing/connection_proxy_spec.rb

@@ -12,6 +12,13 @@ describe Gitlab::Database::LoadBalancing::ConnectionProxy do
   end
 
   describe '#select_all' do
+    let(:override_proxy) { ActiveRecord::Base.connection.class }
+
+    # We can't use :Gitlab::Utils::Override because this method is dynamically prepended
+    it 'method signatures match', :rails5 do
+      expect(proxy.method(:select_all).parameters).to eq(override_proxy.instance_method(:select_all).parameters)
+    end
+
     describe 'using a SELECT query' do
       it 'runs the query on a secondary' do
         arel = double(:arel)