fix(schema): avoid throwing duplicate index error if index spec keys have different order or index has a custom name

[vkarpov15]

Fix #15109

Summary

#15109 points out that if index spec has keys in different order, that is still valid. So schema.index({ a: 1, b: 1 }); schema.index({ b: 1, a: 1 }) should work. Also, if there is a custom index name, then that is fine too, because MongoDB enforces index uniqueness based on name. It's just that two indexes with the same spec will have the same name unless an explicit name option is specified in the index options.

Examples