Encoder: try to use pointer type TextMarshaler (#679)

If a type does not implement the encoding.TextMarshaler interface but
its pointer type does, use it if possible.

Fixes #678

marshaler.go

@@ -209,7 +209,12 @@ func (enc *Encoder) encode(b []byte, ctx encoderCtx, v reflect.Value) ([]byte, e
 		}
 	}
 
-	if v.Type().Implements(textMarshalerType) {
+	hasTextMarshaler := v.Type().Implements(textMarshalerType)
+	if hasTextMarshaler || (v.CanAddr() && reflect.PtrTo(v.Type()).Implements(textMarshalerType)) {
+		if !hasTextMarshaler {
+			v = v.Addr()
+		}
+
 		if ctx.isRoot() {
 			return nil, fmt.Errorf("toml: type %s implementing the TextMarshaler interface cannot be a root element", v.Type())
 		}
@@ -657,7 +662,7 @@ func willConvertToTable(ctx encoderCtx, v reflect.Value) bool {
 	if !v.IsValid() {
 		return false
 	}
-	if v.Type() == timeType || v.Type().Implements(textMarshalerType) {
+	if v.Type() == timeType || v.Type().Implements(textMarshalerType) || (v.Kind() != reflect.Ptr && v.CanAddr() && reflect.PtrTo(v.Type()).Implements(textMarshalerType)) {
 		return false
 	}