Fixed PHP-1111: MongoClient constructor crashes on connection to slave.

https://jira.mongodb.org/browse/PHP-1111
CS -> Connection Slave, CM -> Connection Master.

There is no need to test $master->findOne() as we test in the slave already. If it's in the slave, it should be in master (unless MongoDB is really messed up, and not our problem). 

Of course the first insert fails in line 21, that's why there is a comment "This will fail".

I don't really know why I can read directly from the slave without slaveOkay() actually. Perhaps this is something that the master/slave feature supports? I merely connect to it directly.

I'll fix the variable names, and have a look at that slave read.