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.
Could you use `$that` instead of `$_this`? (just nitpicking)