ng/eval: properly wire through reflect.StructTag

[sbinet]

this fails in ng:

ng> type T struct { N string `json:"name"` }
ng> import "reflect"
ng> reflect.TypeOf(T{}).Field(0).Tag
""

whereas it should display:

ng> import "reflect"
ng> type T struct { N string `json:"name"` }
ng> reflect.TypeOf(T{}).Field(0).Tag
"json:\"name\""