validatorapi: fix content type in http response (ObolNetwork#1803)

Fixes sending incorrect `Content-Type` header in HTTP response. Found this bug while integrating nimbus VC which errored when charon sent `text/plain` instead of `application/json`.

category: bug 
ticket: none

core/validatorapi/router.go

@@ -883,8 +883,6 @@ func getBeaconNodeAddress(ctx context.Context, eth2Cl eth2wrap.Client) (*url.URL
 
 // writeResponse writes the 200 OK response and json response body.
 func writeResponse(ctx context.Context, w http.ResponseWriter, endpoint string, response interface{}) {
- w.WriteHeader(http.StatusOK)
-
  if response == nil {
  return
  }
@@ -952,8 +950,8 @@ func writeError(ctx context.Context, w http.ResponseWriter, endpoint string, err
  log.Error(ctx, "Failed marshalling error response", err2)
  }
 
- w.WriteHeader(aerr.StatusCode)
  w.Header().Set("Content-Type", "application/json")
+ w.WriteHeader(aerr.StatusCode)
 
  if _, err2 = w.Write(b); err2 != nil {
  log.Error(ctx, "Failed writing api error", err2)

core/validatorapi/router_internal_test.go

@@ -210,6 +210,33 @@ func TestRawRouter(t *testing.T) {
  testRawRouter(t, handler, callback)
  })
 
+ t.Run("valid content type in 2xx response", func(t *testing.T) {
+ handler := testHandler{}
+
+ callback := func(ctx context.Context, baseURL string) {
+ res, err := http.Get(baseURL + "/eth/v1/node/version")
+ require.NoError(t, err)
+ require.Equal(t, res.Header.Get("Content-Type"), "application/json")
+ }
+
+ testRawRouter(t, handler, callback)
+ })
+
+ t.Run("valid content type in non-2xx response", func(t *testing.T) {
+ handler := testHandler{}
+
+ callback := func(ctx context.Context, baseURL string) {
+ res, err := http.Post(baseURL+"/eth/v1/validator/duties/attester/1", "", strings.NewReader("not json"))
+ require.NoError(t, err)
+ require.Equal(t, res.Header.Get("Content-Type"), "application/json")
+ var errRes errorResponse
+ require.NoError(t, json.NewDecoder(res.Body).Decode(&errRes))
+ require.Equal(t, errRes.Code, http.StatusBadRequest)
+ }
+
+ testRawRouter(t, handler, callback)
+ })
+
  t.Run("client timeout", func(t *testing.T) {
  cctx, cancel := context.WithCancel(context.Background())
  handler := testHandler{