config() function - fixed $key parameter errors x2 

I have been playing around with testing parts of the framework and I found 2 errors in the config() function. 

The first issue is that it doesn't behave like the function comment where calling config() with no parameters - i.e. $key=null should reset the config.

The 2nd issue throws a php warning in array_merge if a non-string or non-array value for $key is passed. I thought about casting $key to array with (array), but I think it is better to fail safely if $key isn't an array.

I have added some clarifications to readme.md about how the config() routine works.