DE-1143 Use http constants (#356)

events.go

@@ -3,6 +3,7 @@ package mailgun
 import (
 	"context"
 	"fmt"
+	"net/http"
 	"time"
 
 	jsoniter "github.com/json-iterator/go"
@@ -163,7 +164,7 @@ func (ei *EventIterator) fetch(ctx context.Context, url string) error {
 	r.setClient(ei.mg.Client())
 	r.setBasicAuth(basicAuthUser, ei.mg.APIKey())
 
-	resp, err := makeRequest(ctx, r, "GET", nil)
+	resp, err := makeRequest(ctx, r, http.MethodGet, nil)
 	if err != nil {
 		return err
 	}

exports.go

@@ -76,7 +76,7 @@ func (mg *MailgunImpl) GetExportLink(ctx context.Context, id string) (string, er
 
 	r.addHeader("User-Agent", MailgunGoUserAgent)
 
-	req, err := r.NewRequest(ctx, "GET", nil)
+	req, err := r.NewRequest(ctx, http.MethodGet, nil)
 	if err != nil {
 		return "", err
 	}

webhooks_test.go

@@ -141,7 +141,7 @@ func buildFormRequest(fields map[string]string) *http.Request {
 	}
 
 	r := strings.NewReader(values.Encode())
-	req, _ := http.NewRequest("POST", "/", r)
+	req, _ := http.NewRequest(http.MethodPost, "/", r)
 	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
 
 	return req
@@ -157,7 +157,7 @@ func buildMultipartFormRequest(fields map[string]string) *http.Request {
 
 	writer.Close()
 
-	req, _ := http.NewRequest("POST", "/", buf)
+	req, _ := http.NewRequest(http.MethodPost, "/", buf)
 	req.Header.Set("Content-type", writer.FormDataContentType())
 
 	return req