Store the parameters of the chosen connection when using Master/Slave.

When using the MasterSlaveConnection, the internal parameters array has a different structure than the usual Connection. This can cause the following methods to fail: getHost(), getPort(), getUsername() and getPassword().

I changed the MasterSlaveConnection so that it stores the parameters of the chosen connection in a private property. With this change, the methods that used to fail can now retrieve the configuration for the actual chosen connection, which is, in my opinion, clearly the expected behavior.

I chose to create a new private property in MasterSlaveConnection instead of inherit the $_params from Connection in order to not cause any confusion by having two different array structures in the same variable at different situations.

I also expanded the tests for this case.
I think it should. In this very case, the $params['master'] will probably never change, and after connecting to the master once, this line will probably never run again (given the behavior of the current implementation), but the chosenParams property should anyway always carry the parameters of the current connection, so it seems safer to update it accordingly.