Use upstream slack-go/slack again (#1018)

vendor/github.com/slack-go/slack/webhooks_go112.go

@@ -0,0 +1,34 @@
+// +build !go1.13
+
+package slack
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"net/http"
+
+	"github.com/pkg/errors"
+)
+
+func PostWebhookCustomHTTPContext(ctx context.Context, url string, httpClient *http.Client, msg *WebhookMessage) error {
+	raw, err := json.Marshal(msg)
+	if err != nil {
+		return errors.Wrap(err, "marshal failed")
+	}
+
+	req, err := http.NewRequest(http.MethodPost, url, bytes.NewReader(raw))
+	if err != nil {
+		return errors.Wrap(err, "failed new request")
+	}
+	req = req.WithContext(ctx)
+	req.Header.Set("Content-Type", "application/json")
+
+	resp, err := httpClient.Do(req)
+	if err != nil {
+		return errors.Wrap(err, "failed to post webhook")
+	}
+	defer resp.Body.Close()
+
+	return checkStatusCode(resp, discard{})
+}