Fix #967 & made MongoCursor.limit/skip safe

[WebFreak001]

m_currentDoc was relative to the chunk start and not relative to the document start. I dislike the current mongodb testing system by manually calling these test() functions inside unittests when changing something, that basically destroys the whole reason of automated tests. Can't you install mongodb on the travis & appveyor test servers?

[wilzbach]

Can't you install mongodb on the travis & appveyor test servers?

https://github.com/rejectedsoftware/vibe.d/blob/master/.travis.yml

I think the point was that

• not everyone running dub tests has mongo installed and running
• DB tests need fixtures and cleanups, which you would have to add to the unittest as well.

[s-ludwig]

The test() functions in unit tests are actually never invoked at all. Those unit tests just serve as documentation examples that get syntax checked in the unittest build. The actual tests happen as high level tests in the tests/ folder.

[s-ludwig]

I'll move the test to the tests/ folder after merge. The fix looks good!