Deployment configuration updates fail when using IoTHubConfigurationManager.update_configuration

# Context

- **OS and version used:** MacOS 11.6.4
- **Python version:** 3.7.5
- **pip version:** 20.2.3
- **list of installed packages:** azure-iot-hub 2.6.0
## Description of the issue

It is not possible to update a device configuration using IoTHubConfigurationManager.update_configuration. All requests result in `Operation returned an invalid status code 'Conflict'`. In the code example below, I retrieve an existing configuration, change the target_condition and then attempt to update it.

## Code sample exhibiting the issue

```
from msrest.exceptions import HttpOperationError
from azure.iot.hub import IoTHubConfigurationManager

config_service = IoTHubConfigurationManager(iothub_conn_str)

# retrieve an existing deployment
current_config = config_service.get_configuration("my_deployment")
print(current_config)

# change the target condition
current_config.target_condition = "tags.environment='test'"

try:
config_service.update_configuration(current_config, current_config.etag)
except HttpOperationError as ex:
print(ex)
```