Add omitempty to legacy fields in Attachment struct

When using `Blocks` in `Attachment` with `Text` and `Fallback` fields,
slack api returns `invalid_attachments` error.
`Attachment` shouldn't have these legacy fields.

attachments.go

@@ -61,7 +61,7 @@ type ConfirmationField struct {
 // Attachment contains all the information for an attachment
 type Attachment struct {
 	Color    string `json:"color,omitempty"`
-	Fallback string `json:"fallback"`
+	Fallback string `json:"fallback,omitempty"`
 
 	CallbackID string `json:"callback_id,omitempty"`
 	ID         int    `json:"id,omitempty"`
@@ -75,7 +75,7 @@ type Attachment struct {
 	Title     string `json:"title,omitempty"`
 	TitleLink string `json:"title_link,omitempty"`
 	Pretext   string `json:"pretext,omitempty"`
-	Text      string `json:"text"`
+	Text      string `json:"text,omitempty"`
 
 	ImageURL string `json:"image_url,omitempty"`
 	ThumbURL string `json:"thumb_url,omitempty"`

chat_test.go

@@ -97,12 +97,11 @@ func TestPostMessage(t *testing.T) {
 			opt: []MsgOption{
 				MsgOptionAttachments(
 					Attachment{
-						Text:   "text",
 						Blocks: blocks,
 					}),
 			},
 			expected: url.Values{
-				"attachments": []string{`[{"fallback":"","text":"text","blocks":` + blockStr + `}]`},
+				"attachments": []string{`[{"blocks":` + blockStr + `}]`},
 				"channel":     []string{"CXXX"},
 				"token":       []string{"testing-token"},
 			},