Validate options for createIndexes command

Consider the command to build a background index:

db.foo.ensureIndex({bar:1}, {backtround: true}) // typo intentional

A typo in the options like the one above will trigger a foreground index build, with may have a very negative impact on production environments.

It would be nice if we could validate options to the ensureIndex() command, and fail with an error message if invalid options are provided.