{CI} Make recording responses' Content-Length match the body

[jiasli]

Symptom

When the test framework replaces a random name, its length is not preserved:

- appxxxxxxxxxxxx
+ app000003

This make Content-Length header in the response not match the actual body, causing failure due to the new azure-core check Azure/azure-sdk-for-python#20888.

https://dev.azure.com/azure-sdk/public/_build/results?buildId=1216795&view=logs&j=771f6cdf-8d98-5d9b-2fb9-d3a6e9554788&t=0a182f1d-9946-580d-467e-4d4ebd6af32d&l=15587

2021-11-29T04:30:29.8165866Z src/azure-cli/azure/cli/command_modules/appservice/commands.py:37: in _ex_handler
2021-11-29T04:30:29.8166504Z     raise ex
2021-11-29T04:30:29.8167402Z src/azure-cli-core/azure/cli/core/commands/__init__.py:692: in _run_job
2021-11-29T04:30:29.8168382Z     result = cmd_copy(params)
2021-11-29T04:30:29.8169244Z src/azure-cli-core/azure/cli/core/commands/__init__.py:328: in __call__
2021-11-29T04:30:29.8169905Z     return self.handler(*args, **kwargs)
2021-11-29T04:30:29.8170779Z src/azure-cli-core/azure/cli/core/commands/command_operation.py:121: in handler
2021-11-29T04:30:29.8171463Z     return op(**command_args)
2021-11-29T04:30:29.8172321Z src/azure-cli/azure/cli/command_modules/appservice/custom.py:240: in create_webapp
2021-11-29T04:30:29.8173461Z     _fill_ftp_publishing_url(cmd, webapp, resource_group_name, name)
2021-11-29T04:30:29.8174425Z src/azure-cli/azure/cli/command_modules/appservice/custom.py:1170: in _fill_ftp_publishing_url
2021-11-29T04:30:29.8175201Z     profiles = list_publish_profiles(cmd, resource_group_name, name, slot)
2021-11-29T04:30:29.8176163Z src/azure-cli/azure/cli/command_modules/appservice/custom.py:2089: in list_publish_profiles
2021-11-29T04:30:29.8176857Z     for f in content:
2021-11-29T04:30:29.8177521Z _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
2021-11-29T04:30:29.8177910Z 
2021-11-29T04:30:29.8178442Z self = <azure.core.pipeline.transport._requests_basic.StreamDownloadGenerator object at 0x7fe8bc14b460>
2021-11-29T04:30:29.8178854Z 
2021-11-29T04:30:29.8179272Z     def __next__(self):
2021-11-29T04:30:29.8179805Z         internal_response = self.response.internal_response
2021-11-29T04:30:29.8180314Z         try:
2021-11-29T04:30:29.8180787Z             chunk = next(self.iter_content_func)
2021-11-29T04:30:29.8181287Z             if not chunk:
2021-11-29T04:30:29.8181750Z                 raise StopIteration()
2021-11-29T04:30:29.8182219Z             return chunk
2021-11-29T04:30:29.8182684Z         except StopIteration:
2021-11-29T04:30:29.8183171Z             internal_response.close()
2021-11-29T04:30:29.8183628Z             raise StopIteration()
2021-11-29T04:30:29.8184159Z         except requests.exceptions.StreamConsumedError:
2021-11-29T04:30:29.8184674Z             raise
2021-11-29T04:30:29.8185198Z         except requests.exceptions.ChunkedEncodingError as err:
2021-11-29T04:30:29.8185727Z             msg = err.__str__()
2021-11-29T04:30:29.8186456Z             if 'IncompleteRead' in msg:
2021-11-29T04:30:29.8187128Z                 _LOGGER.warning("Incomplete download: %s", err)
2021-11-29T04:30:29.8187708Z                 internal_response.close()
2021-11-29T04:30:29.8188399Z >               raise IncompleteReadError(err, error=err)
2021-11-29T04:30:29.8189756Z E               azure.core.exceptions.IncompleteReadError: ('Connection broken: IncompleteRead(1524 bytes read, 143 more expected)', IncompleteRead(1524 bytes read, 143 more expected))

Change

After this PR, the length is preserved:

- appxxxxxxxxxxxx
+ app000000000003

[yonzhan]

CI

[jiasli]

Went down another path to fix Content-Length header instead (#20541).