append writeheader to fix custom headers (GO-7710) (#16)

http.go

@@ -54,18 +54,14 @@ func LambdaAdapter(l LambdaHandler) func(http.ResponseWriter, *http.Request) {
 				}
 			}
 
-			if resp.Headers == nil {
-				resp.Headers = make(map[string]string)
-			}
-			resp.Headers["Content-Type"] = "application/json"
-			w.WriteHeader(resp.StatusCode)
+			w.Header().Set("Content-Type", "application/json")
 			for k, v := range resp.Headers {
 				w.Header().Set(k, v)
 			}
 			for k, v := range resp.MultiValueHeaders {
 				w.Header().Set(k, strings.Join(v, ","))
 			}
-
+			w.WriteHeader(resp.StatusCode)
 			if _, wErr := w.Write([]byte(resp.Body)); wErr != nil {
 				panic(wErr)
 			}

http_test.go

@@ -29,11 +29,11 @@ func TestLambdaAdapter(t *testing.T) {
 
 	w := httptest.NewRecorder()
 	r := httptest.NewRequest(http.MethodGet, "/test/url/path/var1/var2", nil)
-	r.Header.Set("Content-Type", "application/json")
+	r.Header.Set("Content-Type", "text/plain")
 	g8.LambdaAdapter(l)(w, r)
 
 	assert.Equal(t, http.StatusOK, w.Code)
-	assert.Equal(t, "application/json", w.Header().Get("Content-Type"))
+	assert.Equal(t, "text/plain", w.Header().Get("Content-Type"))
 	assert.Equal(t, "cookie1,cookie2", w.Header().Get("Set-Cookie"))
 	assert.Equal(t, "success", w.Body.String())
 }
@@ -44,7 +44,7 @@ func TestLambdaAdapter_without_content_type(t *testing.T) {
 			return &events.APIGatewayProxyResponse{
 				StatusCode:        http.StatusOK,
 				MultiValueHeaders: map[string][]string{"Set-Cookie": {"cookie1", "cookie2"}},
-				Body:              "success",
+				Body:              `{"message":"success"}`,
 			}, nil
 		},
 		Method:     http.MethodGet,
@@ -59,7 +59,7 @@ func TestLambdaAdapter_without_content_type(t *testing.T) {
 	assert.Equal(t, http.StatusOK, w.Code)
 	assert.Equal(t, "application/json", w.Header().Get("Content-Type"))
 	assert.Equal(t, "cookie1,cookie2", w.Header().Get("Set-Cookie"))
-	assert.Equal(t, "success", w.Body.String())
+	assert.Equal(t, `{"message":"success"}`, w.Body.String())
 }
 
 func TestLambdaAdapter_g8_error(t *testing.T) {