Fix bug in namespace check in MappingDriverChain

[ttk]

This fix improves the checking if a namespace appears in a className by ensuring that the namespace ends in a \. Prior to this fix, the following would occur:

A className such as Doctrine\Tests\Persistence\Mapping\DriverChainEntity would match the namespace: Doctrine\Tests\Persistence\Map, which isn't correct.

Other considerations:

• what if the namespace already ends in a \? is this a valid case? Probably not.
• what if it's a parent namespace. In the above example, this would be Doctrine\Tests\Persistence. Should this match against the className Doctrine\Tests\Persistence\Mapping\DriverChainEntity or not? The current logic would match, but I'm not sure if it should....