Abstract Factories handling is inconsistent with normal Factories

[weierophinney]

See this line: https://github.com/zendframework/zf2/blob/master/library/Zend/ServiceManager/ServiceManager.php#L1040-L1042

Essentially, if your abstract factory returns anything other than an object, the SM moves on to the next abstract factory. This means that if a later AF returns a value, that will be used, or, if whatever the last AF returns, regardless of type, will be returned.

As an example, if we register 3 abstract factories, and each return arrays, the last AF's value will be returned, even though the first clearly returned a value. This is inconsistent with factories, where any non-null value may be returned as a service, which is in turn inconsistent with how setService() works (only arrays and objects are valid).

At the very least, createFromAbstractFactory should return immediately if a non-null value is returned by a give abstract factory. Better yet, we should also loosen up how setService() works, to allow setting the same type range as factories do.

[stefanotorresi]

FWIW I was about to make a PR for this. Shall I?

edit: actually I'll just submit the PR and let you decide what to do with it :p