fix: don't let case-insensitive fallback reuse an exactly-matched key

[koriyoshi2041]

Fixes #75.

When a struct has two fields whose names differ only by case, decoding a map that contains one of them populates both fields:

type Result struct {
    Name string `mapstructure:"name"`
    NAME string `mapstructure:"NAME"`
}
var r Result
mapstructure.Decode(map[string]any{"name": "lower"}, &r)
// got:  {Name:"lower" NAME:"lower"}
// want: {Name:"lower" NAME:""}

In decodeStructFromMap, the Name field matches the key "name" exactly and removes it from dataValKeysUnused. The NAME field has no exact key, so it falls into the case-insensitive search — which iterates all keys and matches "name" again via MatchName, even though it was already consumed. (encoding/json gives the value only to the exact match.)

The fix skips keys that are no longer in dataValKeysUnused during the case-insensitive fallback, so each map key is claimed by at most one field, and the exact match wins.

Added TestDecode_caseInsensitiveDuplicateKey; it fails before the fix (NAME is "lower") and passes after. Existing TestDecode* tests still pass.