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**Apache Airflow version**:
Apache Airflow: 2.0.0

**Kubernetes version (if you are using kubernetes)** (use `kubectl version`):

**Environment**:

- **Cloud provider or hardware configuration**:
- **OS** (e.g. from /etc/os-release): 
Distributor ID: Ubuntu
Description:    Ubuntu 18.04.5 LTS
Release:        18.04
Codename:       bionic
- **Kernel** (e.g. `uname -a`):
Linux Personal 5.4.0-62-generic #70~18.04.1-Ubuntu SMP Tue Jan 12 17:18:00 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux
- **Install tools**:
- **Others**:

**What happened**:

<!-- (please include exact error messages if you can) -->
REST API doesn't return the **extra** field of the connection in the response.

**What you expected to happen**:

<!-- What do you think went wrong? -->
It should return all the fields as shown in the documentation.
![Screenshot from 2021-01-18 20-10-09](https://user-images.githubusercontent.com/15157792/104928902-38224280-59c9-11eb-814a-3f359c0796f2.png)


**How to reproduce it**:
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- Kind https://kind.sigs.k8s.io/docs/user/quick-start/

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Create one connection with id  **leads_ec2** and define values as shown in the screenshot. 

![Screenshot from 2021-01-18 19-49-48](https://user-images.githubusercontent.com/15157792/104927763-d44b4a00-59c7-11eb-9139-da83d7098b3c.png)

Now call the below API endpoint to get the connection details. And as shown in the screenshot it doesn't include the extra field in the response.
**API Endpoint** : `http://localhost:8000/api/v1/connections/leads_ec2`
 
![Screenshot from 2021-01-18 19-50-07](https://user-images.githubusercontent.com/15157792/104928126-491e8400-59c8-11eb-80d5-84b52e812d8e.png)

**How often does this problem occur? Once? Every time etc?**:

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Same for other connection_id. It doesn't return the extra field in the response.

tests/api_connexion/endpoints/test_connection_endpoint.py
response = self.client.get(
            "/api/v1/connections/invalid-connection", environ_overrides={'REMOTE_USER': "test"}
        )
        assert response.status_code == 404
        assert {
            'detail': "The Connection with connection_id: `invalid-connection` was not found",
            'status': 404,
            'title': 'Connection not found',
            'type': EXCEPTIONS_LINK_MAP[404],
        } == response.json
    def test_should_raises_401_unauthenticated(self):
        response = self.client.get("/api/v1/connections/test-connection-id")
        assert_401(response)
class TestGetConnections(TestConnectionEndpoint):
    @provide_session
    def test_should_respond_200(self, session):
        connection_model_1 = Connection(conn_id='test-connection-id-1', conn_type='test_type')
        connection_model_2 = Connection(conn_id='test-connection-id-2', conn_type='test_type')
        connections = [connection_model_1, connection_model_2]
        session.add_all(connections)
        session.commit()
        result = session.query(Connection).all()
        assert len(result) == 2
        response = self.client.get("/api/v1/connections", environ_overrides={'REMOTE_USER': "test"})
        assert response.status_code == 200
        assert response.json == {
            'connections': [
                {
                    "connection_id": "test-connection-id-1",