validatorapi: fix content type in http response (#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{