1.x stable

add an option no_grid_ensure_index.
    
    without this option, gridfs recreates indexes on every file read, and twice on puts
    
    this is due to the fact that the [] method on Mongo::DB instantiates a new collection every time a new
    Mongo::Grid is created. This causes the cache used by ensure_index to be empty every time, in turn
    causing indexes to be rewritten

This patch is only really useful if for some (likely unjustifiable) reason you're creating Mongo::Grid lots of times, but if you are, it's nice to have the option not to spend time ensuring index.