Fix failing test in MySQL.

Signed-off-by: NJosé Valim <jose.valim@gmail.com>

activerecord/test/cases/associations/belongs_to_associations_test.rb

@@ -34,9 +34,14 @@ def test_belongs_to_with_primary_key
 
   def test_belongs_to_with_primary_key_joins_on_correct_column
     sql = Client.joins(:firm_with_primary_key).to_sql
+    if current_adapter?(:MysqlAdapter)
+      assert_no_match /`firm_with_primary_keys_companies`\.`id`/, sql
+      assert_match /`firm_with_primary_keys_companies`\.`name`/, sql
+    else
       assert_no_match /"firm_with_primary_keys_companies"\."id"/, sql
       assert_match /"firm_with_primary_keys_companies"\."name"/, sql
     end
+  end
 
   def test_proxy_assignment
     account = Account.find(1)