func (c *Collection) EnsureIndexKey(key ...string) error {
	return c.EnsureIndex(Index{Key: key})
}
func (c *Collection) EnsureIndexKey(key ...string) error {
	return c.EnsureIndex(Index{Key: key})
}
// EnsureIndexKey ensures an index with the given key exists, creating it
// if necessary.
//
// This example:
//
//     err := collection.EnsureIndexKey("a", "b")
//
// Is equivalent to:
//
//     err := collection.EnsureIndex(mgo.Index{Key: []string{"a", "b"}})
//
// See the EnsureIndex method for more details.