fix instance variable @client not initialized warnings

Author: bvogelzang

test/test_helper.rb

@@ -44,7 +44,7 @@ def sqlserver?
       self.class.sqlserver?
     end
 
-    def close_client(client=@client)
+    def close_client(client=(@client ||= nil))
       client.close if defined?(client) && client.is_a?(TinyTds::Client)
     end