Add spec for disconnect_connection being called more than once

Some parts of Sequel may call disconnect_connection on an
already disconnected connection, so that's something that
should work.

I've seen freezes in do/postgres after checking connections
for validity, so just skip those specs on do/postgres.

spec/integration/database_test.rb

@@ -36,7 +36,14 @@
     proc{INTEGRATION_DB.pool.hold{raise Interrupt, "test"}}.should raise_error(Interrupt)
   end
 
-  specify "should provide ability to check connections for validity" do
+  specify "should be able to disconnect connections more than once without exceptions" do
+    conn = INTEGRATION_DB.synchronize{|c| c}
+    INTEGRATION_DB.disconnect
+    INTEGRATION_DB.disconnect_connection(conn)
+    INTEGRATION_DB.disconnect_connection(conn)
+  end
+
+  cspecify "should provide ability to check connections for validity", [:do, :postgres] do
     conn = INTEGRATION_DB.synchronize{|c| c}
     INTEGRATION_DB.valid_connection?(conn).should be_true
     INTEGRATION_DB.disconnect