webhooks: remove go1.12 support

Signed-off-by: Koichi Shiraishi <zchee.io@gmail.com>

webhooks.go

@@ -1,7 +1,10 @@
 package slack
 
 import (
+	"bytes"
 	"context"
+	"encoding/json"
+	"fmt"
 	"net/http"
 )
 
@@ -35,3 +38,24 @@ func PostWebhookCustomHTTP(url string, httpClient *http.Client, msg *WebhookMess
 func PostWebhookContextCustomHTTP(ctx context.Context, url string, httpClient *http.Client, msg *WebhookMessage) error {
 	return PostWebhookCustomHTTPContext(ctx, url, httpClient, msg)
 }
+
+func PostWebhookCustomHTTPContext(ctx context.Context, url string, httpClient *http.Client, msg *WebhookMessage) error {
+	raw, err := json.Marshal(msg)
+	if err != nil {
+		return fmt.Errorf("marshal failed: %w", err)
+	}
+
+	req, err := http.NewRequestWithContext(ctx, http.MethodPost, url, bytes.NewReader(raw))
+	if err != nil {
+		return fmt.Errorf("failed new request: %w", err)
+	}
+	req.Header.Set("Content-Type", "application/json")
+
+	resp, err := httpClient.Do(req)
+	if err != nil {
+		return fmt.Errorf("failed to post webhook: %w", err)
+	}
+	defer resp.Body.Close()
+
+	return checkStatusCode(resp, discard{})
+}