[11.x] Allow MultipleInstanceManager to have studly creators

[cosmastech]

Manager uses studly case for driver creator names. MultipleInstanceManager just sets the first character of the driver as upper case.

If I have a driver called in_memory, then in my child MultipleInstanceManager, the creator function would look like this:

// Current Implementation
protected function createIn_memoryDriver() { }

// After this PR
protected function createInMemoryDriver() { }

This looks better, brings parity between the two Manager classes, but is not a breaking change because we only check for the studly case method if the ucfirst() formatted method doesn't exist on the class.