Fixed bug of active? for Azure SQL.

I'm poor at English.
If you can't understand what I mean, please let me know.

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Azure SQL close the client connection every 5min.
active? method of TinyTds::Client can't diagnose it.

This is a specification of Azure SQL.
http://blogs.msdn.com/b/bartr/archive/2010/06/18/sql-azure-connection-retry.aspx

As a result, adapter needs execute SQL because of correct checking.