Fix alias serialization for MongoDB

[jotak]

Forward PR #1428 to branch QUERYDSL_3

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Fixes #1427 + add junit tests:

When querying for QFood.food.as(QFish.class).breed.eq("unknown")

I expect to see { "name" : "f1" , "breed" : "unknown"}

rather than { "name" : "f1" , "food.breed" : "unknown"}

Note this is my first commit on querydsl, so I hope I have a correct understanding of what "PathType.DELEGATE" is and how it should be interpreted.

[Shredder121]

Thank you, since the main PR was already accepted we'll only wait for the build to pass.

[johnktims]

Thanks for backporting this fix, @jotak.

[jotak]

you're welcome