fix omitempty panic for uncomparable fields (#57)

Author: alxfv

encoder_test.go

@@ -1534,13 +1534,18 @@ func TestEncoderEmbedModes(t *testing.T) {
 
 func TestOmitEmpty(t *testing.T) {
 
+	type NotComparable struct {
+		Slice []string
+	}
+
 	type Test struct {
 		String  string            `form:",omitempty"`
 		Array   []string          `form:",omitempty"`
 		Map     map[string]string `form:",omitempty"`
 		String2 string            `form:"str,omitempty"`
 		Array2  []string          `form:"arr,omitempty"`
 		Map2    map[string]string `form:"map,omitempty"`
+		NotComparable			  `form:",omitempty"`
 	}
 
 	var tst Test

util.go

@@ -51,6 +51,12 @@ func hasValue(field reflect.Value) bool {
 	case reflect.Slice, reflect.Map, reflect.Ptr, reflect.Interface, reflect.Chan, reflect.Func:
 		return !field.IsNil()
 	default:
-		return field.IsValid() && field.Interface() != reflect.Zero(field.Type()).Interface()
+		if !field.IsValid() {
+			return false
+		}
+		if !field.Type().Comparable() {
+			return true
+		}
+		return field.Interface() != reflect.Zero(field.Type()).Interface()
 	}
 }