Handle duplicates in URI options array

The driver handles duplicate options with different character casing very poorly. For instance, if the options array contained both `readpreference` and `readPreference`, bson_iter_init_find_case() would only pick up the first matching key and ignore subsequent duplicates.

This should be changed to iterate over the options to be consistent with the handling of other duplicates in the URI options array, with the last value overriding previous occurrences regardless of casing.