[Python] Allow Overriding Accept Header

[ckoegel]

The Python SDK currently does not allow for overriding the Accept header at runtime. The header can be set using the _headers argument when calling an operation method, but this gets overwritten by the select_header_accept method called when the data is serialized to a request. This PR adds a conditional that skips the calling of the select_header_accept method if an Accept header has been provided by the user.

For example, calling

response = self.conferences_api_instance.download_conference_recording(
            'accountId, 'conferenceId', 'recordingId', _headers={'Accept': 'audio/vnd.wave'})

with

[
    'audio/vnd.wave', 
    'audio/mpeg', 
    'application/json'
]

as the list of accept headers being passed to the select_header_accept method, would previously set the Accept header to application/json but now respects the audio/vnd.wave passed in by the user.

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[wing328]

CI failure not related to this change