Force an open connection

[heynineteen]

Currently, a command can be executed against a closed connection and that command will succeed. Suggest simulating the real behavior of DbConnections which would throw a System.InvalidOperationException unless the connection is open.

[dvoituron]

For the moment, we do not actually use the connection status to be able to use it. The objective is to simulate the data returns during a call to an SQL database.

But it is an interesting feature.
I propose to add an option to MockDbConnection allowing to trigger an exception when the connection state is not "open".