SERVER-14237: The "size" parameter for non-capped collections stopped being honored in HEAD

The documentation for [db.createCollection](http://docs.mongodb.org/manual/reference/method/db.createCollection/) states the following for the `size` parameter:

> If `capped` is false, you can use this field to preallocate space for an ordinary collection.

This was true prior to `r2.7.1` but the `size` parameter gets silently ignored in `HEAD` when the collection to be created is not capped. It was the only (documented) way not to allocate a lot of extents for a collection that was supposed to grow to a large size, say 500 MiB.