imp: willYield handle Generator::getReturn()

[camilledejoye]

The current implementation of MethodProphecy::willYield() does not allow to specify a return value for the mocked generator.

The proposal here is to add an optional parameter allowing to define the return value:

$prophecy->createGenerator()->willYield(['foo', 'bar'], 'return value');

[ciaranmcnulty]

Looks good, thank you!