Revert "check connection active to prevent usage of dead/closed connection"

This reverts commit 6861763.

Author: mgrunberg

lib/active_record/connection_adapters/sqlserver/database_statements.rb

@@ -464,7 +464,7 @@ def finish_statement_handle(handle)
         end
 
         def ensure_established_connection!
-          raise TinyTds::Error, 'SQL Server client is not connected' if @connection.nil? || !@connection.active?
+          raise TinyTds::Error, 'SQL Server client is not connected' unless @connection
 
           yield
         end