Marshal panics on struct containing reflect.Value (upstream #346)

[xe-nvdk]

Upstream: vmihailenco#346

Marshalling a struct that contains reflect.Value in a map[string]interface{} causes a nil pointer dereference panic in field.EncodeValue(). The encoder does not handle reflect.Value types safely.

[efegokdemir]

I am reproducing the reflect.Value panic against the current v6 branch before deciding whether a focused fix is still needed.

[efegokdemir]

Implemented in PR #87. The encoder now recognizes reflect.Value before generic struct traversal and returns the normal unsupported-value error instead of panicking when it is nested in an interface map. Added a regression test. Validation passed: go test ./..., go test -race ./..., go vet ./..., and git diff --check.