* make TTL indexes behave like other indexes on creation
if a TTL index is already present on a collection, the previous behavior
was to make subsequent calls to `ensureIndex` fail unconditionally with
the error "there can only be one ttl index per collection".
now, we are comparing the attributes of the to-be-created index with the
attributes of the existing TTL index and make it only fail when the
attributes differ. if the attributes are identical, the `ensureIndex`
call succeeds and returns the existing index.
* added tests
* updated CHANGELOG