fix-#842: Fix ignored chained options

[DavidBord]

fixes #842

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Repository health increased by 0.07% when pulling 9a1fe6b on DavidBord:fix-842 into 1e930fe on MongoEngine:master.

• 1 new problem was found (including 0 errors and 1 code smell).
• 5 problems were fixed (including 0 errors and 5 code smells).

[MRigal]

Thanks @DavidBord
Change assertIsNone for old Python 2.6 and check maybe if we need at index creation (and therefore maybe somewhere else, for mongoengine <2.8), to force call the cursor generation....

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Repository health increased by 0.07% when pulling cea09b3 on DavidBord:fix-842 into 1e930fe on MongoEngine:master.

• 1 new problem was found (including 0 errors and 1 code smell).
• 5 problems were fixed (including 0 errors and 5 code smells).