Support valid index options with invalid value types for legacy restores

In the linked HELP ticket, there's a case where the expireAfterSeconds index option has a string value rather than the expected numeric from a 3.6 mongodump (e.g. "1234" instead of 1234).

We should investigate how this might've happened, and see if we should support numeric strings for this index option (and possibly other index options) when restoring with --convertLegacyIndexes.

This would be a relatively simple fix using the strconv package for example.