Fix handling of empty value. (#104)

If a value is decoding into a type that implements the
encoding.TextUnmarshaler interface, the decoder should use it's
UnmarshalText method in all instances including an empty value.

Previously, would ignore the method decoding the field as the empty
value of the type.

Author: emil2k

Diff for: decoder.go

@@ -283,11 +283,7 @@ func (d *Decoder) decode(v reflect.Value, path string, parts []pathPart, values
 			val = values[len(values)-1]
 		}
 
-		if val == "" {
-			if d.zeroEmpty {
-				v.Set(reflect.Zero(t))
-			}
-		} else if conv != nil {
+		if conv != nil {
 			if value := conv(val); value.IsValid() {
 				v.Set(value.Convert(t))
 			} else {
@@ -321,6 +317,10 @@ func (d *Decoder) decode(v reflect.Value, path string, parts []pathPart, values
 					}
 				}
 			}
+		} else if val == "" {
+			if d.zeroEmpty {
+				v.Set(reflect.Zero(t))
+			}
 		} else if conv := builtinConverters[t.Kind()]; conv != nil {
 			if value := conv(val); value.IsValid() {
 				v.Set(value.Convert(t))

Diff for: decoder_test.go

@@ -1737,3 +1737,33 @@ func TestTextUnmarshalerTypeSliceOfStructs(t *testing.T) {
 		t.Fatal("Expecting invalid path error", err)
 	}
 }
+
+type S22 string
+
+func (s *S22) UnmarshalText(text []byte) error {
+	*s = S22("a")
+	return nil
+}
+
+// Test to ensure that when a field that should be decoded into a type
+// implementing the encoding.TextUnmarshaler interface is set to an empty value
+// that the UnmarshalText method is utilized over other methods of decoding,
+// especially including simply setting the zero value.
+func TestTextUnmarshalerEmpty(t *testing.T) {
+	data := map[string][]string{
+		"Value": []string{""}, // empty value
+	}
+	// Implements encoding.TextUnmarshaler, should use the type's
+	// UnmarshalText method.
+	s := struct {
+		Value S22
+	}{}
+	decoder := NewDecoder()
+	if err := decoder.Decode(&s, data); err != nil {
+		t.Fatal("Error while decoding:", err)
+	}
+	expected := S22("a")
+	if expected != s.Value {
+		t.Errorf("Expected %v errors, got %v", expected, s.Value)
+	}
+}