**Apache Airflow version**: 2.0.0

**What happened**:

Using endpoint `/dags/{dag_id}/dagRuns/{dag_run_id}/taskInstances/{task_id}/xcomEntries/{xcom_key}` I got Response Body but without `value` entry. Like:
```
{
  "dag_id": "string",
  "execution_date": "string",
  "key": "string",
  "task_id": "string",
  "timestamp": "string"
}
```
Instead of:
```
{
  "dag_id": "string",
  "execution_date": "string",
  "key": "string",
  "task_id": "string",
  "timestamp": "string",
  "value": "string"
}
```
The exact value by defined `key` exists.
tests/api_connexion/endpoints/test_xcom_endpoint.py
#
#
import unittest
from datetime import timedelta
from parameterized import parameterized
from airflow.models import DagModel, DagRun as DR, XCom
from airflow.security import permissions
from airflow.utils.dates import parse_execution_date
from airflow.utils.session import provide_session
from airflow.utils.types import DagRunType
from airflow.www import app
from tests.test_utils.api_connexion_utils import assert_401, create_user, delete_user
from tests.test_utils.config import conf_vars
from tests.test_utils.db import clear_db_dags, clear_db_runs, clear_db_xcom
class TestXComEndpoint(unittest.TestCase):