adding in softer check for content type (#14097)

validator/client/beacon-api/json_rest_handler.go

@@ -109,7 +109,7 @@ func decodeResp(httpResp *http.Response, resp interface{}) error {
 		return errors.Wrapf(err, "failed to read response body for %s", httpResp.Request.URL)
 	}
 
-	if httpResp.Header.Get("Content-Type") != api.JsonMediaType {
+	if !strings.Contains(httpResp.Header.Get("Content-Type"), api.JsonMediaType) {
 		// 2XX codes are a success
 		if strings.HasPrefix(httpResp.Status, "2") {
 			return nil

validator/client/beacon-api/json_rest_handler_test.go

@@ -99,7 +99,16 @@ func Test_decodeResp(t *testing.T) {
 	type j struct {
 		Foo string `json:"foo"`
 	}
-
+	t.Run("200 JSON with charset", func(t *testing.T) {
+		body := bytes.Buffer{}
+		r := &http.Response{
+			Status:     "200",
+			StatusCode: http.StatusOK,
+			Body:       io.NopCloser(&body),
+			Header:     map[string][]string{"Content-Type": {"application/json; charset=utf-8"}},
+		}
+		require.NoError(t, decodeResp(r, nil))
+	})
 	t.Run("200 non-JSON", func(t *testing.T) {
 		body := bytes.Buffer{}
 		r := &http.Response{