Test(client): Add dead test after closing client

We recently encountered an unexpected Segmentation Fault Error when
checking whether a client was dead after closing it.

To consider this in the future, I've updated the test cases for the
client to include this scenario, so we can catch any issues earlier on.

Author: Matias Martini

test/client_test.rb

@@ -16,6 +16,7 @@ class ClientTest < TinyTds::TestCase
       assert @client.close
       assert @client.closed?
       assert !@client.active?
+      assert @client.dead?
       action = lambda { @client.execute('SELECT 1 as [one]').each }
       assert_raise_tinytds_error(action) do |e|
         assert_match %r{closed connection}i, e.message, 'ignore if non-english test run'