Error "unsupported type; maybe include it the UnmarshalText interface or register it using custom type?" with map[string]*struct

[darigaaz]

Code to reproduce:

func TestMapToPtrStruct(t *testing.T) {
	s := struct {
		// pointer to struct
		M map[string]*struct {
			ID string
		}
	}{}

	vals := url.Values{
		"M[key].ID": []string{"M[key].ID"},
	}

	dec := formam.NewDecoder(nil)
	if err := dec.Decode(vals, &s); err != nil {
		t.Fatalf("error when decode %s", err)
	}

	if v, ok := s.M["key"]; !ok {
		t.Error("The key \"key\" in M does not exists")
	} else if v.ID != "M[key].ID" {
		t.Error("The value in key \"key\" of M is incorrect")
	}
}

Expected: no error
Actual: error when decode formam: unsupported type; maybe include it the UnmarshalText interface or register it using custom type?